graphics/gegl: fix build on non-OpenMP platforms

Previously, this was doubly worked around. On armv7 by disabling
OpenMP and in general by compiling with gcc. Unfortunately, using
gcc caused GeGL to fail at runtime with symbol errors like this one
in games/toppler:

GEGL-Message: 10:22:16.282: Module '/usr/local/lib/gegl-0.4/gegl-common-cxx.so' load error: /usr/local/lib/gegl-0.4/gegl-common-cxx.so: Undefined symbol "__aeabi_d2lz@GCC_3.5"
GEGL operation missing!

Avoid this problem by not using gcc and instead disabling OpenMP on all
platforms.

Approved by: portmgr (build fix blanket)
MFH: 2025Q2

(cherry picked from commit 8ccc484526b042307cf8668884b4c8a6a8c7ea51)