predicate macros for building on Windows

With this changeset, building gr1c for Windows is operational but not
fully tested. To do it, first install a cross-compiler from the
mingw-w64 project (http://mingw-w64.org/). Major GNU/Linux
distributions have this available in respective package repositories;
e.g., on Fedora or RedHat, `dnf install mingw64-gcc`, and on Ubuntu,
`apt-get install gcc-mingw-w64`. Then,

    export CC=x86_64-w64-mingw32-gcc
    export LD="x86_64-w64-mingw32-ld -r"

Then, for building CUDD,

    ./configure --prefix=`pwd`/../.. --host=x86_64-w64-mingw32

and for gr1c,

    make -e

This is part of work for issue #34
and tulip-control/tulip-control#197

* about _WIN32 and _WIN64 macros, https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=vs-2019
* about _spawnvp(), https://docs.microsoft.com/en-us/cpp/c-runtime-library/spawn-wspawn-functions?view=vs-2019

src/main.c

@@ -11,6 +11,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <ctype.h>
+#if defined(_WIN32) || defined(_WIN64)
+#include <process.h>
+#endif
 
 #include "common.h"
 #include "logging.h"
@@ -94,43 +97,55 @@ int main( int argc, char **argv )
 
             /* Pass arguments after rg */
             command_argv = malloc( sizeof(char *)*argc );
-            command_argv[0] = strdup( "gr1c rg" );
+            command_argv[0] = strdup( "\"gr1c rg\"" );
             command_argv[argc-1] = NULL;
             for (i = 1; i < argc-1; i++)
                 command_argv[i] = argv[i+1];
 
+#if defined(_WIN32) || defined(_WIN64)
+            return _spawnvp( _P_WAIT, "gr1c-rg", command_argv );
+#else
             if (execvp( "gr1c-rg", command_argv ) < 0) {
                 perror( __FILE__ ",  execvp" );
                 return -1;
             }
+#endif
 
         } else if (!strncmp( argv[1], "patch", strlen( "patch" ) )
                    && argv[1][strlen("patch")] == '\0') {
 
             command_argv = malloc( sizeof(char *)*argc );
-            command_argv[0] = strdup( "gr1c patch" );
+            command_argv[0] = strdup( "\"gr1c patch\"" );
             command_argv[argc-1] = NULL;
             for (i = 1; i < argc-1; i++)
                 command_argv[i] = argv[i+1];
 
+#if defined(_WIN32) || defined(_WIN64)
+            return _spawnvp( _P_WAIT, "gr1c-patch", command_argv );
+#else
             if (execvp( "gr1c-patch", command_argv ) < 0) {
                 perror( __FILE__ ",  execvp" );
                 return -1;
             }
+#endif
 
         } else if (!strncmp( argv[1], "autman", strlen( "autman" ) )
                    && argv[1][strlen("autman")] == '\0') {
 
             command_argv = malloc( sizeof(char *)*argc );
-            command_argv[0] = strdup( "gr1c autman" );
+            command_argv[0] = strdup( "\"gr1c autman\"" );
             command_argv[argc-1] = NULL;
             for (i = 1; i < argc-1; i++)
                 command_argv[i] = argv[i+1];
 
+#if defined(_WIN32) || defined(_WIN64)
+            return _spawnvp( _P_WAIT, "gr1c-autman", command_argv );
+#else
             if (execvp( "gr1c-autman", command_argv ) < 0) {
                 perror( __FILE__ ",  execvp" );
                 return -1;
             }
+#endif
 
         } else if (!strncmp( argv[1], "help", strlen( "help" ) )
                    && argv[1][strlen("help")] == '\0') {