lavfi/xbr: undef PI if defined

This conflict with the DJGPP libc math.h which includes a PI macro (to
M_PI).

We cannot make DJGPP POSIX only (using -D_POSIX_SOURCE) to avoid this
kind of symbols conflicts due to the lack of both posix_memalign and
memalign (DJGPP non standard function) in that POSIX mode. We currently
rely on memalign for aligned heap allocation.

libavfilter/vf_xbr.c

@@ -37,6 +37,10 @@
 #define RED_BLUE_MASK 0x00FF00FF
 #define GREEN_MASK    0x0000FF00
 
+#ifdef PI
+#undef PI
+#endif
+
 typedef int (*xbrfunc_t)(AVFilterContext *ctx, void *arg, int jobnr, int nb_jobs);
 
 typedef struct {