build using CFLAGS passed in at configure

In order to build xfsprogs in a hermetic build, we need be able to
pass in -I and -L flags to the compiler and linker, respectively.
This needs to be used by the configure script, but we also need to
make sure these flags are used by the Makefiles as well.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Christoph Hellwig <hch@lst.de>