Clean-up some CMake

- some image libraries define _INCLUDE_DIR and not INCLUDE_DIRS.
- CMakePushCheckState is a non-destructive way of changing the
CMAKE_REQUIRED_* variables.

Change-Id: I7d318a0ddf9754a5c047f47ba1713f9f94e1ec1c

cmake/cpu.cmake

@@ -54,12 +54,14 @@ endif()
 list(LENGTH WEBP_SIMD_FLAGS WEBP_SIMD_FLAGS_LENGTH)
 math(EXPR WEBP_SIMD_FLAGS_RANGE "${WEBP_SIMD_FLAGS_LENGTH} - 1")
 
+include(CMakePushCheckState)
 foreach(I_SIMD RANGE ${WEBP_SIMD_FLAGS_RANGE})
   list(GET WEBP_SIMD_FLAGS ${I_SIMD} WEBP_SIMD_FLAG)
 
   # First try with no extra flag added as the compiler might have default flags
   # (especially on Android).
   unset(WEBP_HAVE_${WEBP_SIMD_FLAG} CACHE)
+  cmake_push_check_state()
   set(CMAKE_REQUIRED_FLAGS)
   webp_check_compiler_flag(${WEBP_SIMD_FLAG} ${WEBP_ENABLE_SIMD})
   if(NOT WEBP_HAVE_${WEBP_SIMD_FLAG})
@@ -116,4 +118,5 @@ foreach(I_SIMD RANGE ${WEBP_SIMD_FLAGS_RANGE})
       endif()
     endif()
   endif()
+  cmake_pop_check_state()
 endforeach()