run-command: conditionally define locate_in_PATH()

This commit doesn't change any behaviour by itself, but allows us to easily
define compat replacements for locate_in_PATH(). It prepares us for the next
commit that adds a native Windows implementation of locate_in_PATH().

Signed-off-by: Matthias Aßhauer <mha1993@live.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -170,6 +170,7 @@ int is_executable(const char *name)
 	return st.st_mode & S_IXUSR;
 }
 
+#ifndef locate_in_PATH
 /*
  * Search $PATH for a command.  This emulates the path search that
  * execvp would perform, without actually executing the command so it
@@ -218,6 +219,7 @@ static char *locate_in_PATH(const char *file)
 	strbuf_release(&buf);
 	return NULL;
 }
+#endif
 
 int exists_in_PATH(const char *command)
 {