ensure M_PI is defined

Some image manipulation functions use trigonometric functions.
This includes using the number Pi.  Pi is often available as
M_PI via including <math.h>.  This constant is part of the Unix98
standard, not the C standard itself.  According to the GCC
documentation[1], M_PI is only defined when the feature selection
macro _XOPEN_SOURCE=500 is used.  This seems to be default in
many versions of GNU/Linux, since ssocr could always use M_PI
without explicitly setting _XOPEN_SOURCE=500.

An MPlayer bug report[2] reported a build failure because M_PI
was not defined.  This lead to a patch[3] to work around this
problem.

I want to avoid running into this specific problem in the future,
without introducing significant changes to ssocr.  Thus I check
if M_PI is defined after inlcuding <math.h>, and define it myself
if it is missing.  I use the value found in /usr/include/math.h
from GNU Libc on my current system (Ubuntu GNU/Linux 20.04.6 LTS).

[1] https://www.gnu.org/software/libc/manual/html_node/Mathematical-Constants.html
[2] https://trac.mplayerhq.hu/ticket/2423
[3] https://lists.mplayerhq.hu/pipermail/mplayer-dev-eng/2024-December/074244.html

Also add '+' to the version number to indicate changes after the
latest release.

defines.h

@@ -23,7 +23,7 @@
 #define SSOCR2_DEFINES_H
 
 /* version number */
-#define VERSION "2.24.1"
+#define VERSION "2.24.1+"
 
 /* states */
 #define FIND_DARK 0

imgproc.c

@@ -27,8 +27,11 @@
 /* string manipulation */
 #include <string.h>         /* strcasecmp, strcmp, strrchr */
 
-/* sin, cos */
-#include <math.h>
+/* trigonometry */
+#include <math.h>           /* sin, cos, M_PI */
+#ifndef M_PI                /* sometimes, M_PI is not defined */
+#define M_PI 3.14159265358979323846
+#endif
 
 /* my headers */
 #include "defines.h"        /* defines */