Apparently older gcc's handle macros with variable arguments slightly
differently.  From this section of the gcc manual for the version I am
using, I suspect the offhand comment here has actually been implemented:

===
   It might be better to discard the last preprocessor token instead of
the last preceding sequence of non-whitespace characters; in fact, we
may someday change this feature to do so.  We advise you to write the
macro definition so that the preceding sequence of non-whitespace
characters is just a single token, so that the meaning will not change
if we change the definition of this feature.
===

Anyway, for macros with variable arguments I had to change the last ``foo,
##stuff'' to ``foo , ##stuff''.  That is, put a space before the comma.

The following is a demonstration:

diff -ru gst-plugins-0.7.1.orig/ext/ffmpeg/gstffmpegcodecmap.c
gst-plugins-0.7.1/ext/ffmpeg/gstffmpegcodecmap.c
--- gst-plugins-0.7.1.orig/ext/ffmpeg/gstffmpegcodecmap.c       2003-07-07
14:29:16.000000000 -0700
+++ gst-plugins-0.7.1/ext/ffmpeg/gstffmpegcodecmap.c    2003-10-27
13:42:35.000000000 -0800
@@ -44,7 +44,7 @@
                      "height", GST_PROPS_INT (context->height),\
                      "framerate", GST_PROPS_FLOAT (            \
                                1.*context->frame_rate/         \
-                               context->frame_rate_base),      \
+                               context->frame_rate_base) ,     \
                      ##props)                                  \
        :                                                       \
        GST_CAPS_NEW (name,                                     \
@@ -52,7 +52,7 @@
                      "width",  GST_PROPS_INT_RANGE (16, 4096), \
                      "height", GST_PROPS_INT_RANGE (16, 4096), \
                      "framerate", GST_PROPS_FLOAT_RANGE (0,    \
-                                               G_MAXFLOAT),    \
+                                               G_MAXFLOAT) ,   \
                      ##props)
 
 /* same for audio - now with channels/sample rate


I've not finished building, so not sure if that patch is sufficient, yet. 
But if I don't post this bug now, I'll probably forget about it.

Oh yeah, gcc version 2.95.4 20020723 (prerelease)