configure.ac: Remove AC_ISC_POSIX macro; it's broken on some platforms and not needed.

configure.ac: Remove AC_ISC_POSIX macro; it's broken on some platforms and not needed.

Original commit message from CVS:
* configure.ac:
Remove AC_ISC_POSIX macro; it's broken on some platforms and not needed.
Add check (taken from -base) for winsock, adds WIN32_LIBS
* gst/Makefile.am:
Add WIN32_LIBS to LIBADD for libgstreamer. Needed now that gstpoll uses
winsock.
Define GST_EXPORTS when building libgstreamer (only used on win32)
* gst/gst_private.h:
* gst/gstinfo.h:
Use GST_EXPORT instead of locally-defined (and incorrect IMPORT_SYMBOL)
for symbols that we need to export in both these files.
* gst/gstpoll.c:
Include gst_private.h higher up to avoid some compile problems on win32.