[AD] errno_t and strerror_s non-standard types - cmake incorrectly detects strerror_s when not present

[Edgar Reynaldo <edgarreynaldo@xxxxxxxxxx>]

I recently updated cmake to the latest version (3.7.1) after finding out 
that cmake is incorrectly detecting strerror_s in MinGW 5.3.0 when it 
doesn't exist in MinGW.

errno_t and strerror_s are used in file_stdio.c on line 302 when 
ALLEGRO_HAVE_STRERROR_S is defined. It is defined by a check for 
"check_function_exists(strerror_s ALLEGRO_HAVE_STRERROR_S)" on line 486 
of CMakeLists.txt. strerror_s and errno_t are nowhere to be found in 
MinGW as indicated by a comment in file_stdio.c on line 289.

Not sure how to fix this, but currently Allegro fails to build on MinGW 
because of the faulty detection by CMake.

Edgar