Fix missing definitions in Glib C 2.27

This close #1

src/libcephes/mconf.h

@@ -72,13 +72,26 @@ Copyright 1984, 1987, 1989, 1995 by Stephen L. Moshier
 /* Constant definitions for math error conditions
  */
 
-/* Already defined in math.h */
-/* #define DOMAIN		1	|+ argument domain error +| */
-/* #define SING		2	|+ argument singularity +| */
-/* #define OVERFLOW	3	|+ overflow range error +| */
-/* #define UNDERFLOW	4	|+ underflow range error +| */
-/* #define TLOSS		5	|+ total loss of precision +| */
-/* #define PLOSS		6	|+ partial loss of precision +| */
+/* These constants were defined in math.h, prior to G libc 2.27 */
+#ifndef DOMAIN
+#define DOMAIN		1	/* argument domain error */
+#endif
+#ifndef SING
+#define SING		2	/* argument singularity */
+#endif
+#ifndef OVERFLOW	
+#define OVERFLOW	3	/* overflow range error */
+#endif
+#ifndef UNDERFLOW	
+#define UNDERFLOW	4	/* underflow range error */
+#endif
+#ifndef TLOSS		
+#define TLOSS		5	/* total loss of precision */
+#endif
+#ifndef PLOSS		
+#define PLOSS		6	/* partial loss of precision */
+#endif
+
 #define TOOMANY         7       /* too many iterations */
 #define MAXITER        500