Skip find_package for OpenGL when not needed. Link again to OpenGL::G…

…L when using Emscripten.

Backends/CMakeLists.txt

@@ -76,6 +76,10 @@ if(UNIX)
 	# This is required in some UNIX and UNIX-like operating systems to load shared object files at runtime
 	target_link_libraries(rmlui_backend_SDL_GL3 INTERFACE ${CMAKE_DL_LIBS})
 endif()
+if(EMSCRIPTEN)
+	# Only Emscripten requires linking to OpenGL::GL, for other platforms we use 'glad' as an OpenGL loader.
+	target_link_libraries(rmlui_backend_SDL_GL3 INTERFACE OpenGL::GL)
+endif()
 
 add_library(rmlui_backend_SDL_VK INTERFACE)
 target_sources(rmlui_backend_SDL_VK INTERFACE

CMake/DependenciesForBackends.cmake

@@ -163,7 +163,9 @@ if(RMLUI_BACKEND MATCHES "GL2$")
 	report_dependency_found_or_error("OpenGL" OpenGL::GL)
 endif()
 
-if(RMLUI_BACKEND MATCHES "GL3$")
+# We use 'glad' as an OpenGL loader for GL3 backends, thus we don't normally need to link to OpenGL::GL. The exception
+# is for Emscripten, where we use a custom find module to provide OpenGL support.
+if(EMSCRIPTEN AND RMLUI_BACKEND MATCHES "GL3$")
 	find_package("OpenGL" "3")
 	report_dependency_found_or_error("OpenGL" OpenGL::GL)
 endif()