WIP Changes to support building for OS/2

+ WIP Makefile updates to support building for OS/2
   - TODO: Makefile needs SYSNAME test to detect OS/2 (Bill has asked Elbert)
 + Added "__declspec(dllexport)" (via new "TX_EXTERN" #define'd in
   `tp_magic_api.h`) to public function definitions in Magic tools
   - WIP: So far just "Mirror/Flip" plugin
   - TODO: Add "TX_EXTERN" prefix to all other public Magic tool functions
 + Renamed long Magic tool source files to have shorter filenames
   - WIP: So far just "Mirror/Flip" (`mirror_flip.c` -> `mirror_f.c`)
   - TODO: Rename all other magic tool source files.

h/t Elbert Pol for providing info and some diffs!

src/fonts.c

@@ -19,7 +19,7 @@
   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
   (See COPYING.txt)
 
-  Last modified: June 29, 2023
+  Last modified: June 6, 2024
 */
 
 #include <stdio.h>
@@ -101,7 +101,7 @@ static const char *problemFontExtensions[] = {
 #include <sys/poll.h>
 #include <sys/wait.h>
 
-#ifdef _POSIX_PRIORITY_SCHEDULING
+#if defined(_POSIX_PRIORITY_SCHEDULING) && !defined(__OS2__)
 #include <sched.h>
 #else
 #define sched_yield()