libcap: Configure Make variables correctly without a horrible hack

Occasionally, the build would fail with:

  make[2]: execvp: mkdir: Argument list too long

This turned out to be due to a hacky solution used in the recipe to
modify the Makefile, which resulted in one more $(BUILD_CFLAGS) being
added to the immediately expanded BUILD_CFLAGS Make variable each time
do_configure was executed. After a couple of times, this lead to an
environment with a 140 kB BUILD_CFLAGS when mkdir should execute, which
resulted in the E2BIG.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>