cmake: Don't abort if not testing

This prevents installing and is quite antisocial.

CMakeLists.txt

@@ -11,7 +11,7 @@ add_definitions(-D_CRT_SECURE_NO_WARNINGS)
 
 option(GLM_TEST_ENABLE "GLM test" OFF)
 if(NOT GLM_TEST_ENABLE)
-	message(FATAL_ERROR "GLM is a header only library, no need to build it. Set the option GLM_TEST_ENABLE with ON to build and run the test bench")
+	message(STATUS "GLM is a header only library, no need to build it. Set the option GLM_TEST_ENABLE with ON to build and run the test bench")
 endif()
 
 if(("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang") OR ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU") OR (("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Intel") AND UNIX))