Remove build hacks for FreeBSD.

* configure.ac [freebsd]: Do not add /usr/local to CPPFLAGS and
LDFLAGS.

Back in ~2000 we introduced a quick hack to make building of Libgcrypt
on FreeBSD easier by always adding -I/usr/local/include and
-L/usr/local/lib . It turned out that this is a bad idea if one wants
to build with library version which is not installed in /usr/local.
The hack made was eventually (in 2003) copied from Libgcrypt to
GnuPG-2.