FindOpenSSL: Fix finding header files and static libraries on MinGW

* Avoid searching in the `Program Files` folder because the official
  OpenSSL is built for the MSVC ABI, and so is not compatible with MinGW.

* Static libraries on MinGW has `.a` extension.

Modules/FindOpenSSL.cmake

@@ -210,7 +210,7 @@ endif ()
 # Support preference of static libs by adjusting CMAKE_FIND_LIBRARY_SUFFIXES
 if(OPENSSL_USE_STATIC_LIBS)
   set(_openssl_ORIG_CMAKE_FIND_LIBRARY_SUFFIXES ${CMAKE_FIND_LIBRARY_SUFFIXES})
-  if(WIN32)
+  if(MSVC)
     set(CMAKE_FIND_LIBRARY_SUFFIXES .lib .a ${CMAKE_FIND_LIBRARY_SUFFIXES})
   else()
     set(CMAKE_FIND_LIBRARY_SUFFIXES .a )
@@ -230,7 +230,7 @@ else()
   set(_OPENSSL_FIND_PATH_SUFFIX "include")
 endif()
 
-if (WIN32)
+if (MSVC)
   # http://www.slproweb.com/products/Win32OpenSSL.html
   set(_OPENSSL_ROOT_HINTS
     ${OPENSSL_ROOT_DIR}