TECHNICAL FIELD The present invention relates to a front-engine, rear-wheel-drive car. BACKGROUND ART High-performance, front-engine sports cars normally have a rear-wheel or permanent four-wheel drive. The most widely used rear-wheel drive solution features a front clutch and gearbox in series with the drive shaft of the engine; the output of the gearbox is connected to a propeller shaft extending longitudinally to the rear axle where it is connected to a differential for distributing drive torque to the two rear drive wheels. Though widely adopted, by being easy to produce and a good compromise between performance and comfort, the above design solution has various drawbacks : unbalanced weight distribution towards the front axle, a relatively high centre of gravity of the car, and the need for an ample engine compartment. DISCLOSURE OF INVENTION It is an object of the present invention to provide a front-engine, rear-wheel-drive car which is cheap and easy to produce and, at the same time, eliminates the aforementioned drawbacks. According to the present invention, there is provided a front-engine, rear-wheel-drive car, as recited in the accompanying Claims. BRIEF DESCRIPTION OF THE DRAWINGS A number of non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a schematic plan view of a front- engine, rear-wheel-drive car in accordance with the present invention; Figure 2 shows an operating diagram of a dual-clutch gearbox of the Figure 1 car; Figure 3 shows a simplified operating diagram of the dual-clutch gearbox in Figure 2; Figure 4 shows, schematically, the spatial arrangement of some of the components of the Figure 2 dual-clutch gearbox. BEST MODE FOR CARRYING OUT THE INVENTION Number 1 in Figure 1 indicates a car comprising two front driven wheels 2; and two rear drive wheels 3, to which a drive torque, produced by a front longitudinal internal combustion engine 4, is transmitted. More specifically, engine 4 comprises eight cylinders 5 arranged in two rows and connected mechanically to a drive shaft 6, which extends longitudinally, i.e. parallel to the travelling direction of car 1, and transmits drive torque to rear drive wheels 3 by means of a power train 7. Power train 7 comprises a longitudinal one-piece tubular propeller shaft 8 connecting drive shaft 6 to a rear clutch-gearbox assembly 9. Clutch-gearbox assembly 9 incorporates a dual clutch 10 (in particular, an oil-bath disk clutch) and, in series, a dual-clutch gearbox 11, which transmits drive torque to a self-locking differential 12, which in turn distributes the drive torque to the two rear drive wheels 3 by means of two axle shafts 13. Consequently, gearbox 11 is connected directly to differential 12, and propeller shaft 8 is connected permanently to drive shaft 6 at one end, and permanently to the input of dual clutch 10 at the other end. As shown in Figures 2, 3 and 4, the input of dual clutch 10 is connected permanently to propeller shaft 8, while the two outputs of dual clutch 10 are connected respectively to two primary shafts 14 and 15 of gearbox 11, which are nested coaxially one inside the other, so that, when dual clutch 10 is operated, each primary shaft 14, 15 is connectable selectively to propeller shaft 8 to receive the drive torque produced by engine 4. Each primary shaft 14, 15 supports two gears 16 angularly integral with primary shaft 14, 15. Gearbox 11 also comprises two secondary shafts 17 and 18, each supporting four gears 19 connected in rotary manner to respective secondary shaft 17, 18 and which are selectively locked angularly to respective secondary shaft 17, 18 by means of four common synchronizing devices 20. It should be stressed that each synchronizing device 20 is common to two gears 19. Each gear 19 meshes with a gear 16 with a given gear ratio corresponding to a given speed gear of car 1. The Roman numerals in Figures 2 and 3 indicate the speed gears corresponding to gears 19 (reverse gear is indicated RW) . It should be pointed out that each gear 16 meshes directly with relative gear 19, with the exception of the reverse gear 16, which meshes with relative gear 19 with the interposition of a known idler (not shown) for inverting rotation of secondary shaft 18. As shown in Figures 2 and 4, each secondary shaft 17, 18 supports a gear 21, which is angularly integral with secondary shaft 17, 18 and connected to a gear 22, angularly integral with an input shaft 23 of differential 12, via the interposition of a further gear 24. It is important to note that primary shafts 14, 15, secondary shafts 17, 18, and input shaft 23 are all longitudinal. In actual use, when a primary shaft 14 or 15 is connected to propeller shaft 8 by dual clutch 10, and one of gears 19 is locked angularly to a secondary shaft 17 or 18 by relative synchronizing device 20, power is transmitted from the primary shaft 14, 15 to the secondary shaft 17, 18 by gear 16 meshing with gear 19, and, downstream, is transmitted by secondary shaft 17, 18 to input shaft 23 of differential 12 by gears 21, 22 and 24. Car 1 as described above has numerous advantages : easy manufacture; optimum weight balance between the front and rear axles; relatively low centre of gravity of car 1; and distribution of the mechanical part between the front compartment (housing engine 4) and the rear compartment (housing clutch-gearbox assembly 9) . In a further embodiment not shown, clutch-gearbox assembly 9 as described above may also be used in a car 1 with a central engine 4 (i.e. between the front and rear axles) or a rear engine 4. In both which cases, the only substantial difference in power train 7 is the length of propeller shaft 8 connecting drive shaft 6 to dual clutch 10. That is, given the location of engine 4 immediately adjacent to clutch-gearbox assembly 9, propeller shaft 8 is much shorter or even eliminated (i.e. drive shaft 6 is connected to dual clutch 10 by a gear drive) .