Linux CMake improvements:

* Disable warning output during configuration.
* Allow users to turn on warnings for ffmpeg and other internal projects.
* Add a new variable for easier ffmpeg version migration if we ever
* pursue that.

CMake/CMakeMacros.cmake

@@ -31,11 +31,13 @@ function(sm_add_link_flag target flag)
 endfunction()
 
 function(disable_project_warnings projectName)
-  if (MSVC)
-    sm_add_compile_flag(${projectName} "/W0")
-  elseif(APPLE)
-    set_target_properties(${projectName} PROPERTIES XCODE_ATTRIBUTE_GCC_WARN_INHIBIT_ALL_WARNINGS "YES")
-  else()
-    set_target_properties(${projectName} PROPERTIES COMPILE_FLAGS "-w")
+  if (NOT WITH_EXTERNAL_WARNINGS)
+    if (MSVC)
+      sm_add_compile_flag(${projectName} "/W0")
+    elseif(APPLE)
+      set_target_properties(${projectName} PROPERTIES XCODE_ATTRIBUTE_GCC_WARN_INHIBIT_ALL_WARNINGS "YES")
+    else()
+      set_target_properties(${projectName} PROPERTIES COMPILE_FLAGS "-w")
+    endif()
   endif()
 endfunction()