configure: Warn that cairo-gl and cairo-glesv2 are risky

Most distros do not ship with gl-enabled cairo, since doing so can
result in libgl being linked to each cairo-using client, even if they
don't actually use GL, and this can cause much larger per-client memory
footprint, and thus can become a resource issue.

Furthermore, while in theory this should work fine, we don't actively
test this configuration, and there could be random undiscovered bugs if
it's used.  We keep the option available for people interested in
helping us chase down those issues, but warn everyone else away.

configure.ac

@@ -240,9 +240,11 @@ if test "x$with_cairo" = "ximage"; then
 else
 if test "x$with_cairo" = "xgl"; then
 	cairo_modules="cairo-gl"
+	AC_MSG_WARN([The --with-cairo=gl option can cause increased resource usage and potential instability, and thus is not recommended.  It is needed only for a few special demo programs.])
 else
 if test "x$with_cairo" = "xglesv2"; then
 	cairo_modules="cairo-glesv2"
+	AC_MSG_WARN([The --with-cairo=gles2 option can cause increased resource usage and potential instability, and thus is not recommended.  It is needed only for a few special demo programs.])
 else
 	AC_ERROR([Unknown cairo renderer requested])
 fi