[CMake] Require opengl when building with cocoa

Closes root-project#7160, as discussed in the issue thread.

cmake/modules/SearchInstalledSoftware.cmake

@@ -38,6 +38,9 @@ if(cocoa)
   if(APPLE)
     set(x11 OFF CACHE BOOL "Disabled because cocoa requested (${x11_description})" FORCE)
     set(builtin_freetype ON CACHE BOOL "Enabled because needed for Cocoa graphics (${builtin_freetype_description})" FORCE)
+    if(NOT opengl)
+      message(FATAL_ERROR "Option \"cocoa=ON\" requires \"opengl=ON\"!")
+    endif()
   else()
     message(STATUS "Cocoa option can only be enabled on MacOSX platform")
     set(cocoa OFF CACHE BOOL "Disabled because only available on MacOSX (${cocoa_description})" FORCE)
@@ -549,6 +552,8 @@ if(opengl)
   if(NOT OPENGL_FOUND OR NOT OPENGL_GLU_FOUND)
     if(fail-on-missing)
       message(FATAL_ERROR "OpenGL package (with GLU) not found and opengl option required")
+    elseif(cocoa)
+      message(FATAL_ERROR "OpenGL package (with GLU) not found and opengl option required for \"cocoa=ON\"")
     else()
       message(STATUS "OpenGL (with GLU) not found. Switching off opengl option")
       set(opengl OFF CACHE BOOL "Disabled because OpenGL (with GLU) not found (${opengl_description})" FORCE)