Add cross-platform fl_close_fd(int fd) to close a file descriptor

FL/fl_utf8.h

@@ -179,6 +179,9 @@ FL_EXPORT int fl_open(const char *fname, int oflags, ...);
 
 FL_EXPORT int fl_open_ext(const char *fname, int binary, int oflags, ...);
 
+/* Portable wrapper around unix-style close() function */
+FL_EXPORT int fl_close_fd(int fd);
+
 /* OD: Portable UTF-8 aware unlink wrapper */
 FL_EXPORT int fl_unlink(const char *fname);
 

src/fl_utf8.cxx

@@ -354,7 +354,7 @@ int fl_putenv(const char* var) {
   \param[in] oflags other arguments are as in the standard open() function
   \return    a file descriptor upon successful completion, or -1 in case of error.
 
-  \see fl_fopen(), fl_open_ext().
+  \see fl_fopen(), fl_open_ext(), fl_close_fd(int fd).
 */
 int fl_open(const char* fname, int oflags, ...) {
   int pmode;
@@ -365,6 +365,14 @@ int fl_open(const char* fname, int oflags, ...) {
   return Fl::system_driver()->open(fname, oflags, pmode);
 }
 
+/** Cross-platform function to close a file descriptor.
+ \return 0 in case of success, or -1 in case of error.
+ */
+
+int fl_close_fd(int fd) {
+  return Fl::system_driver()->close_fd(fd);
+}
+
 /** Cross-platform function to open files with a UTF-8 encoded name.
 
   In comparison with fl_open(), this function allows to control whether