Support for MSVC. Use _WIN32 instead of __MINGW32__

This is not a search and replace of __MING32__ with _WIN32.
There are places where __MINGW32__ is still used and all changes are
tested with MSVC 2019 compiler. Although, this commit alone will not work
on MSVC because there are plenty of other things to be done which are
seperated into multiple PR. PR #289 is the one that "works" on MSVC and
unix as well. But that contains too many changes which shouldn't in a single
PR. Even though this PR alone won't compile GRASS GIS on MSVC, it sure will
not break existing compilers which I think is very important.

Complete support for MSVC will be ready after 2/3 PRs

OSGeo#289

Co-authored-by: Rashad Kanavath <[email protected]>

Author: echoix

include/grass/gis.h

@@ -85,7 +85,7 @@ static const char *GRASS_copyright UNUSED = "GRASS GNU GPL licensed Software";
 
 /*! \brief Cross-platform Newline Character */
 #define NEWLINE '\n'
-#ifdef __MINGW32__
+#ifdef _WIN32
 #define HOST_NEWLINE "\r\n"
 #else
 #define HOST_NEWLINE "\n"
@@ -228,7 +228,7 @@ static const char *GRASS_copyright UNUSED = "GRASS GNU GPL licensed Software";
 
 /* Cross-platform Directory Separator Character and null device stuff */
 #define GRASS_DIRSEP '/'
-#ifdef __MINGW32__
+#ifdef _WIN32
 #define HOST_DIRSEP '\\'
 #define G_DEV_NULL  "NUL:"
 #else
@@ -634,7 +634,7 @@ typedef float FCELL;
 #if HAVE_INT64_T
 #include <sys/types.h>
 typedef int64_t grass_int64;
-#elif defined(__MINGW32__)
+#elif defined(_WIN32)
 typedef __int64 grass_int64;
 #elif HAVE_LONG_LONG_INT
 typedef long long int grass_int64;