meson: fail when EGL is requested, but EGL is not available

Closes: #243

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>

meson.build

@@ -184,7 +184,12 @@ if with_egl
    if cc.has_header('epoxy/egl.h', dependencies: epoxy_dep) and epoxy_dep.get_pkgconfig_variable('epoxy_has_egl') == '1'
       gbm_dep = dependency('gbm', version: '>= ' + _gbm_ver, required: require_egl)
       have_egl = gbm_dep.found()
-      conf_data.set('HAVE_EPOXY_EGL_H', 1)
+      if (have_egl)
+         conf_data.set('HAVE_EPOXY_EGL_H', 1)
+      else
+         assert(not require_egl,
+                'egl was explicitely requested which requires gbm, and this is not available')
+      endif
    else
       assert(not require_egl,
              'egl was explicitely requested but it is not supported by epoxy')