Fix VPATH builds from clean checkout.

Fix VPATH builds from clean checkout.

When doing a VPATH build from a fully clean checkout, the GOB outputs
end up in the build directory.  When this happens, #include "pcx.h"
won't find the header because this header is in srcdir.

The solution is to make sure we always #include relative to the top
src/ directory, which is already explicitly added to the compiler
search path.