Preserve previous $CFLAGS for liboil

If the existing CFLAGS are clobbered and replaced with
-DHAVE_SYMBOL_UNDERSCORE, then a 64-bit compiler will be used on
a 64-bit system even if a 32-bit compiler has been specified. This
causes the build to fail when it tries to compile 32-bit assembly
language.
Merge pull request #14 from ptomato/patch-liboil

Preserve previous $CFLAGS for liboil