This fixes a regression from 450c3588;
after testing for -Wl,-z,defs, CFLAGS was restored from an
incorrect variable, leaving CFLAGS essentially empty.

This would break builds for e.g. arm with Clang, where -mthumb would
be essential for being able to build some inline assembly snippets
correctly.

This also had the effect of dropping the default "-g -O2" arguments
from CFLAGS.
Signed-off-by: Martin Storsjö <martin@martin.st>