[FR] Support for advanced compiler option

Custombuild still doesn't support custom:
- CCVER
- CC
- CXX
- CPP
- CFLAGS
- LDFLAGS

This is especially important on FreeBSD where most people will not use the old compiler that comes with the OS, but will use a version coming from ports.

Rarely anyone does that on FreeBSD.

Well, the default gcc on FreeBSD 8 is v4.2, which doesn't have flags for modern processors, so you have to use a more recent gcc if you want to run optimized code or simply want to compile faster.

Here is the official doc:
http://www.freebsd.org/doc/en/articles/custom-gcc/article.html