cmake: Fix rebuilding error with stale cache files

check_c_source_compiles would generate wrong files if the user
does not set correct variables initially even given correct values
later. Protect against this scenario.

This should fix #418.

Also remove "-MT" flags for MSVC which seems to do no good here.

Author: xlz

cmake_modules/FindTurboJPEG.cmake

@@ -38,16 +38,15 @@ FIND_LIBRARY(TurboJPEG_LIBRARIES
     lib64
 )
 
+IF(TurboJPEG_INCLUDE_DIRS AND TurboJPEG_LIBRARIES)
 INCLUDE(CheckCSourceCompiles)
-if(MSVC)
-  set(CMAKE_REQUIRED_DEFINITIONS -MT)
-endif()
 set(CMAKE_REQUIRED_INCLUDES ${TurboJPEG_INCLUDE_DIRS})
 set(CMAKE_REQUIRED_LIBRARIES ${TurboJPEG_LIBRARIES})
 check_c_source_compiles("#include <turbojpeg.h>\nint main(void) { tjhandle h=tjInitCompress(); return 0; }" TURBOJPEG_WORKS)
 set(CMAKE_REQUIRED_DEFINITIONS)
 set(CMAKE_REQUIRED_INCLUDES)
 set(CMAKE_REQUIRED_LIBRARIES)
+ENDIF()
 
 INCLUDE(FindPackageHandleStandardArgs)
 FIND_PACKAGE_HANDLE_STANDARD_ARGS(TurboJPEG DEFAULT_MSG TurboJPEG_LIBRARIES TurboJPEG_INCLUDE_DIRS TURBOJPEG_WORKS)