build: add -Mdse to PGI optimisation flags

Enable dead store elimination.  The last few releases work no worse
with this flag than without.  Older versions failed to build some
source files when using this flag.

Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -2329,7 +2329,7 @@ probe_cc(){
     elif $_cc -V 2>&1 | grep -q Portland; then
         _type=pgi
         _ident="PGI $($_cc -V 2>&1 | awk '/^pgcc/ { print $2; exit }')"
-        opt_common='-alias=ansi -Mlre -Mpre'
+        opt_common='-alias=ansi -Mdse -Mlre -Mpre'
         _cflags_speed="-O3 -Mautoinline -Munroll=c:4 $opt_common"
         _cflags_size="-O2 -Munroll=c:1 $opt_common"
         _cflags_noopt="-O1"