Fix ninja Emscripten build (#648)

Author: mcbarton

cmake/modules/GoogleTest.cmake

@@ -22,10 +22,18 @@ include(ExternalProject)
 if (EMSCRIPTEN)
   if (CMAKE_C_COMPILER MATCHES ".bat")
     set(config_cmd emcmake.bat cmake)
-    set(build_cmd emmake.bat make)
+    if(CMAKE_GENERATOR STREQUAL "Ninja")
+      set(build_cmd emmake.bat ninja)
+    else()
+      set(build_cmd emmake.bat make)
+    endif()
   else()
     set(config_cmd emcmake cmake)
-    set(build_cmd emmake make)
+    if(CMAKE_GENERATOR STREQUAL "Ninja")
+      set(build_cmd emmake ninja)
+    else()
+      set(build_cmd emmake make)
+    endif()
   endif()
 else()
   set(config_cmd ${CMAKE_COMMAND})