Bug 247132

After 255909@main it's possible for the GBM dependency to be disabled by mistake if it's not present in the buildroot. That's too fragile: people building WebKitGTK are not experts on its build dependencies, and we need to make sure no functionality gets disabled unintentionally when working on packaging. One option is to add a build flag to turn off GBM and fail the build if GBM is missing and the flag is not passed. Alternatively, we could make GBM mandatory if USE_OPENGL_OR_ES is ON (since the USE_OPENGL_OR_ES setting determines the value of the ENABLE_WEBGL setting). For example:

diff --git a/Source/cmake/OptionsGTK.cmake b/Source/cmake/OptionsGTK.cmake
index c8772180ceb8..af306a7fd268 100644
--- a/Source/cmake/OptionsGTK.cmake
+++ b/Source/cmake/OptionsGTK.cmake
@@ -393,11 +393,17 @@ if (USE_OPENGL_OR_ES)
         # make libdrm a requirement and define the USE_LIBGBM and USE_TEXTURE_MAPPER_DMABUF
         # macros. When not available, ANGLE will be used in slower software-rasterization mode.
         find_package(GBM)
-        if (GBM_FOUND)
-            find_package(LibDRM REQUIRED)
-            SET_AND_EXPOSE_TO_BUILD(USE_LIBGBM TRUE)
-            SET_AND_EXPOSE_TO_BUILD(USE_TEXTURE_MAPPER_DMABUF TRUE)
+        if (NOT GBM_FOUND)
+            message(FATAL_ERROR "GBM is required for USE_OPENGL_OR_ES")
         endif ()
+
+        find_package(LibDRM)
+        if (NOT LIBDRM_FOUND)
+            message(FATAL_ERROR "libdrm is required for USE_OPENGL_OR_ES")
+        endif ()
+
+        SET_AND_EXPOSE_TO_BUILD(USE_LIBGBM TRUE)
+        SET_AND_EXPOSE_TO_BUILD(USE_TEXTURE_MAPPER_DMABUF TRUE)
     endif ()
 endif ()