Allow for platform-specific core.* config settings

In the Git for Windows project, we have ample precendent for config
settings that apply to Windows, and to Windows only.

Let's formalize this concept by introducing a platform_core_config()
function that can be #define'd in a platform-specific manner.

This will allow us to contain platform-specific code better, as the
corresponding variables no longer need to be exported so that they can
be defined in environment.c and be set in config.c

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -202,6 +202,11 @@ static int ask_yes_no_if_possible(const char *format, ...)
 	}
 }
 
+int mingw_core_config(const char *var, const char *value)
+{
+	return 0;
+}
+
 int mingw_unlink(const char *pathname)
 {
 	int ret, tries = 0;

compat/mingw.h

@@ -11,6 +11,9 @@ typedef _sigset_t sigset_t;
 #undef _POSIX_THREAD_SAFE_FUNCTIONS
 #endif
 
+extern int mingw_core_config(const char *var, const char *value);
+#define platform_core_config mingw_core_config
+
 /*
  * things that are not available in header files
  */

config.c

@@ -1114,7 +1114,7 @@ static int git_default_core_config(const char *var, const char *value)
 	}
 
 	/* Add other config variables here and to Documentation/config.txt. */
-	return 0;
+	return platform_core_config(var, value);
 }
 
 static int git_default_i18n_config(const char *var, const char *value)

git-compat-util.h

@@ -330,6 +330,14 @@ extern char *gitdirname(char *);
 #define _PATH_DEFPATH "/usr/local/bin:/usr/bin:/bin"
 #endif
 
+#ifndef platform_core_config
+static inline int noop_core_config(const char *var, const char *value)
+{
+	return 0;
+}
+#define platform_core_config noop_core_config
+#endif
+
 #ifndef has_dos_drive_prefix
 static inline int git_has_dos_drive_prefix(const char *path)
 {