gmp: improved gmp by using '-fexceptions' for all stages besides cross stage

Note:

The authors of ppl advise the user to use '-fexceptions'.

configure output of the ppl package:
----------------------------------------------------------------------------
configure: WARNING: CANNOT PROPAGATE EXCEPTIONS BACK FROM GMP:
*** MEMORY EXHAUSTION MAY RESULT IN ABRUPT TERMINATION.
*** This is OK, if you do not plan to use the bounded memory capabilities
*** offered by the PPL.  Otherwise, if you are using GCC or the Intel C/C++
*** compiler, please make sure you use a version of GMP compiled with the
*** `-fexceptions' compiler option.
*** To build such a version, you can configure GMP as follows:
*** CPPFLAGS=-fexceptions ./configure --enable-cxx --prefix=/usr/local
----------------------------------------------------------------------------

base/gmp/gmp.conf

@@ -2,7 +2,7 @@
 # This copyright note is auto-generated by ./scripts/Create-CopyPatch.
 #
 # Filename: package/.../gmp/gmp.conf
-# Copyright (C) 2007 - 2009 The OpenSDE Project
+# Copyright (C) 2007 - 2010 The OpenSDE Project
 # Copyright (C) 2004 - 2006 The T2 SDE Project
 #
 # More information can be found in the files COPYING and README.
@@ -20,7 +20,10 @@ if atstage toolchain; then
 fi
 
 # at gmp priority in cross stage we do not have a g++ available
-atstage cross || var_append extraconfopt ' ' '--enable-cxx'
+if ! atstage cross; then
+	var_append extraconfopt ' ' '--enable-cxx'
+	export CPPFLAGS="-fexceptions"
+fi
 
 var_append confopt ' ' "--enable-mpfr"