mark cmake project explicitly as C++ (see #309)

CMakeLists.txt

@@ -16,7 +16,7 @@ endif()
 # Project configuration
 #
 
-project(EnTT VERSION 3.2.0)
+project(EnTT VERSION 3.2.0 LANGUAGES CXX)
 
 include(GNUInstallDirs)
 
@@ -191,6 +191,10 @@ if(BUILD_TESTING)
     option(BUILD_MOD "Build mod example." OFF)
     option(BUILD_SNAPSHOT "Build snapshot example." OFF)
 
+    if(BUILD_MOD)
+        enable_language(C)
+    endif()
+
     enable_testing()
     add_subdirectory(test)
 endif()