Log Message:
Include all buildlink3.mk before compiler.mk, as the buildlink dependencies
may alter the compiler selection.

In this particular case, librevenge depends on boost-libs, which will
set GCC_REQD and possibly select a GCC dependency, so long as the compiler
hasn't been forcibly pre-selected by including compiler.mk too early.

Fixes build of libwpg on platforms with a native GCC older than what
boost-libs calls for.