Find post-build.sh even when not building in .

You can make configure build in a different directory than the current
one when you give it a `--prefix` option. When doing that, the current
directy will be the build directory, not the source directory. This
breaks running `./post-build.sh`.

This commit fixes it by replacing the invocation with something that
takes into account `$VPATH` which properly references the source
directory. The `post-build.sh` still works properly because it
references the current directory to do its thing, which will (correctly)
be the build directory.

makefile.am

@@ -132,5 +132,11 @@ test_tests_SOURCES=\
     test/src/test_affine.c \
     test/src/test_bezier.c
 
+# When running configure with --prefix, $VPATH references
+# the source directory that post-build.sh is in. When not
+# using a prefix, $VPATH will be unset, so we need to fall
+# back to using . to run the script.
+export VPATH
+
 all-local:
-	sh ./post-build.sh
+	sh $${VPATH:-.}/post-build.sh