Recover accidentally deleted code

src/config.in.hpp

@@ -60,6 +60,9 @@
 /* Defined to 1 if the underlying system provides the strtof function. */
 #cmakedefine HAVE_STRTOF 1
 
+/* Defined to 1 if the underlying system provides the M_PI constant. */
+#cmakedefine HAVE_M_PI 1
+
 /* Defined to 1 if the underlying system provides the posix_fadvise function. */
 #cmakedefine HAVE_POSIX_FADVISE 1
 
@@ -126,6 +129,11 @@ typedef int pid_t;
 inline float strtof( const char *s, char **se ) { return (float) strtod( s, se ); }
 #endif
 
+#if !defined(HAVE_M_PI)
+/* This is only called if the math header files don't include it: stating it here is fine. */
+#define M_PI 3.1415926535897932384626433832795
+#endif
+
 /* Define standard endianness macros, if they're missing. */
 #if defined(HAVE_ENDIAN_H)
 #include <endian.h>