don't override CFLAGS

don't override CFLAGS

leveldb adds -I flags to CFLAGS and CXXFLAGS, but if these macros are
overridden in the make command line, the flags are dropped, and the
build fails.  leveldb should probably use AM_CFLAGS instead, but the
spec file can specify the preferred CFLAGS in the configure command
line, and then everything will work as expected.

Signed-off-by: Alexandre Oliva <oliva@lsd.ic.unicamp.br>
Signed-off-by: Sage Weil <sage.weil@dreamhost.com>