configure.ac: Detect and indicate if GCC inline assembly syntax is available.

configure.ac: Detect and indicate if GCC inline assembly syntax is available.

Original commit message from CVS:
* configure.ac:
Detect and indicate if GCC inline assembly syntax is
available.
* gst/goom/Makefile.am:
* gst/goom/convolve_fx.c:
* gst/goom/flying_stars_fx.c:
* gst/goom/goom_config.h:
* gst/goom/goom_core.c:
* gst/goom/goomsl.c:
* gst/goom/ifs.c:
* gst/goom/mmx.c:
* gst/goom/plugin_info.c:
* gst/goom/xmmx.c:
Fix various GCC-isms, and only build the inline assembly
with compilers that support GCC inline assembly.
Fix a couple of other warnings shown with Forte.