check for process.h, declares getpid() on Windows

check for process.h, declares getpid() on Windows

Original commit message from CVS:

* configure.ac:
* win32/common/config.h:
* win32/common/config.h.in:
check for process.h, declares getpid() on Windows
* gst/gstinfo.c:
include process.h if we have it
* gst/gstmemchunk.c: (populate), (gst_mem_chunk_new):
* gst/gstmemchunk.h:
fix signedness issues
* win32/common/libgstreamer.def:
fix get_type's