mesa: aid users needing libglvnd

This does a simple check during build time for libglvnd and
enables it if present. This prevents users from having to fork
the package to add GL support.

I want to /eventually/ add some kind of "optional" dependency
feature to the package manager so we can be rid of these hacks
once and for all.

extra/mesa/build

@@ -33,6 +33,11 @@ export CFLAGS="$CFLAGS -DGLX_X86_READONLY_TEXT"
 sed "/pre_args += '-DUSE_ELF_TLS'/d" meson.build > _
 mv -f _ meson.build
 
+# To prevent the need for users to fork the mesa package to add
+# libglvnd support, the below code checks for its availability
+# and enables it if present. ie: install glvnd, rebuild mesa.
+! kiss l libglvnd >/dev/null 2>&1 || glvnd_enabled=true
+
 meson \
     --prefix=/usr \
     --sysconfdir=/etc \
@@ -42,6 +47,7 @@ meson \
     -Dplatforms=wayland \
     -Dglx=disabled \
     -Dzstd=false \
+    -Dglvnd="${glvnd_enabled:-false}"
     . output
 
 ninja -C output