build: sanitize CFLAGS handling

As for LDFLAGS previously: leave the CFLAGS "user variable" untouched to
enable easy selective overriding of the flags used by the build system.

Using $lt_prog_compiler_pic shouldn't be necessary, let's drop it and
see if anything breaks. WERROR_CFLAGS was undefined, so dropped as well.

Signed-off-by: Ferenc Wágner <wferi@debian.org>