Allow using shared fmt

Note that FMT < 6.0.0 uses exceptions, which is explicitly disabled when
building dolphin. For example, using unmodified 5.3.0 fails to compile.

CMakeLists.txt

@@ -532,7 +532,15 @@ if (_M_X86)
   add_subdirectory(Externals/Bochs_disasm)
 endif()
 add_subdirectory(Externals/cpp-optparse)
-add_subdirectory(Externals/fmt EXCLUDE_FROM_ALL)
+
+find_package(fmt 6.0.0)
+if(fmt_FOUND)
+  message(STATUS "Using shared fmt")
+else()
+  message(STATUS "Using static fmt from Externals")
+  add_subdirectory(Externals/fmt EXCLUDE_FROM_ALL)
+endif()
+
 add_subdirectory(Externals/glslang)
 add_subdirectory(Externals/imgui)