[lazarus] fpcmake and fpc source dependency

[Peter Vreman]

The reason why you need to have the fpc source tree is that when packages
(rtl/fcl/paszlib/etc.) are installed for a specific target you don't have all information
available for other targets. In the source tree all this information is available.

The reason why not all information is included in Package.fpc is that it will then also
require that you install the units for all targets. An example is that the fcl for linux
will require the inet unit, but this unit is not available in win32. So creating a
Makefile with dependency on fcl that supports linux under win32 is not possible as it
can't find the information for inet.