Link mpfr dynamically

Index: gdb/Makefile.in
--- gdb/Makefile.in.orig
+++ gdb/Makefile.in
@@ -195,7 +195,7 @@ LIBBABELTRACE = @LIBBABELTRACE@
 LIBIPT = @LIBIPT@
 
 # Where is libmpfr?  This will be empty if libmpfr was not available.
-LIBMPFR = @LIBMPFR@
+LIBMPFR = @LTLIBMPFR@
 
 WARN_CFLAGS = @WARN_CFLAGS@
 WERROR_CFLAGS = @WERROR_CFLAGS@