python: fix in-source python builds

the Makefile for the python bindings does a check for out of source
builds to see if it should copy the .pyx file. However, if `srcdir` and
`builddir` are different strings but point to the same directory, the
`cp` command will fail.

This patch makes the build step more robust by checking if the .pyx file
exists in the `builddir` (will resolve to true if srcdir==builddir).
Otherwise, it will copy the file over.

Signed-off-by: Caetano Melone <melone1@llnl.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>

Author: cmelone

src/python/Makefile.am

@@ -36,7 +36,7 @@ EXTRA_DIST = libseccomp.pxd seccomp.pyx setup.py
 all-local: build
 
 build: ../libseccomp.la libseccomp.pxd seccomp.pyx setup.py
-	[ ${srcdir} = ${builddir} ] || cp ${srcdir}/seccomp.pyx ${builddir}
+	[ -f ${builddir}/seccomp.pyx ] || cp ${srcdir}/seccomp.pyx ${builddir}
 	${PY_BUILD} && touch build
 
 install-exec-local: build