Check for __ARM_NEON__ for libpng (#679)

*  Added check to see if __ARM_NEON__ is defined. Now we can use the following command: cmake --build --config Release ..

* Rename to use_arm_neon.cmake to check_if_neon_available.cmake for clarity, minor tidying up of script, and simplifying try_compile() code for ARM NEON.

dlib/CMakeLists.txt

@@ -385,7 +385,10 @@ if (NOT TARGET dlib)
                external/zlib/uncompr.c
                external/zlib/zutil.c
                )
-            if (NEON)
+
+            include(cmake_utils/check_if_neon_available.cmake)
+            if (NEON OR COMPILER_ON_ARM_NEON)
+               message ("NEON will be used for libpng.")
                enable_language(ASM)
                set(source_files ${source_files}
                   external/libpng/arm/arm_init.c

dlib/cmake_utils/check_if_neon_available.cmake

+# This script checks if __ARM_NEON__ is defined for your compiler
+
+cmake_minimum_required(VERSION 2.8.12)
+
+# Don't rerun this script if its already been executed.
+if (DEFINED COMPILER_ON_ARM_NEON)
+   return()
+endif()
+
+# Set to false unless we find out otherwise in the code below.
+set(COMPILER_ON_ARM_NEON 0)
+
+# test if __ARM_NEON__ is defined
+try_compile(test_for_neon_worked ${PROJECT_BINARY_DIR}/neon_test_build ${CMAKE_CURRENT_LIST_DIR}/test_for_neon 
+	neon_test)
+
+message ("ARM NEON TEST OK: ${test_for_neon_worked}")
+
+if(test_for_neon_worked)
+	message ("__ARM_NEON__ defined.")
+	set(COMPILER_ON_ARM_NEON 1)
+endif()

dlib/cmake_utils/test_for_neon/CMakeLists.txt

+
+cmake_minimum_required(VERSION 2.8.12)
+project(neon_test)
+
+add_library(neon_test STATIC neon_test.cpp )
+

dlib/cmake_utils/test_for_neon/neon_test.cpp

+#ifdef __ARM_NEON__
+#else
+#error "No NEON"
+#endif
+int main(){}
+
+
+// ------------------------------------------------------------------------------------
+