issue doxygen#11416 CMake: Error in doc_internal/CMakeLists.txt:22 Problem configuring file

Based on the comment doxygen#11416 (comment)
Made definitions of `M_PI` and `strcasecmp` consistent between `gd.h` and `mscgen_config.h`

Author: albert-github

deps/libmscgen/mscgen_config.h

@@ -2,15 +2,22 @@
 #define MSCGEN_CONFIG_H
 
 #if defined(_WIN32) && !defined(__CYGWIN__)
-#define M_PI   3.14159265358979323846264338327950288
-#define strcasecmp _stricmp
 #define strdup _strdup
 #define fileno _fileno
 #define YY_NO_UNISTD_H 1
 #else
 #define HAVE_UNISTD_H 1
 #endif
 
+#ifndef M_PI
+# define M_PI 3.14159265358979323846
+#endif
+#ifdef _MSC_VER
+#  ifndef strcasecmp
+#    define strcasecmp _stricmp
+#  endif 
+#endif
+
 /* Define to 1 if you have the <limits.h> header file. */
 #define HAVE_LIMITS_H 1