Enable debug info when CC_FOR_BUILD is gcc.

* aclocal.m4 (GMP_PROG_CC_FOR_BUILD): Add -g when compiling with
gcc.

ChangeLog

+2017-10-16  Niels Möller  <nisse@lysator.liu.se>
+
+	* aclocal.m4 (GMP_PROG_CC_FOR_BUILD): Add -g when compiling with
+	gcc.
+
 2017-09-24  Niels Möller  <nisse@lysator.liu.se>
 
 	* tools/pkcs1-conv.c (base64_decode_in_place): New helper

aclocal.m4

@@ -526,7 +526,7 @@ else
     fi
   fi
   if test "$CC_FOR_BUILD" = gcc ; then
-    CC_FOR_BUILD="$CC_FOR_BUILD -O"
+    CC_FOR_BUILD="$CC_FOR_BUILD -O -g"
   fi
 fi