Link mpfr dynamically

Index: gdb/Makefile.in
--- gdb/Makefile.in.orig
+++ gdb/Makefile.in
@@ -199,7 +199,7 @@ LIBXXHASH = @LIBXXHASH@
 LIBIPT = @LIBIPT@
 
 # Where is libmpfr?  This will be empty if libmpfr was not available.
-LIBMPFR = @LIBMPFR@
+LIBMPFR = @LTLIBMPFR@
 
 # GNU source highlight library.
 SRCHIGH_LIBS = @SRCHIGH_LIBS@