Fix for gmp/Makefile, problems occur when /bin/sh is not bash

[Bernie Pope]

Hi,

I'm away from a development machine, so I can't commit the change  
myself. Apologies for that.

We should avoid bash features which are not valid in ordinary /bin/sh.

This popped up while I was building from the latest stable source  
6.8.2 on Solaris.

See the diff below:

diff Makefile.old Makefile

67c67,68
<           export PATH=`pwd`:$$PATH; \
---
 >           PATH=`pwd`:$$PATH; \
 >           export PATH; \
79c80,81
<           export PATH=`pwd`:$$PATH; \
---
 >           PATH=`pwd`:$$PATH; \
 >           export PATH; \