Trac #16149: sage-env exports empty CXXFLAGS

That happens if `CFLAGS` are empty (or not exported at all, which
presumably is the most frequent case) and `CXXFLAGS` aren't either:

{{{
#!sh
if [ "$CXXFLAGS" = "" ]; then
    export CXXFLAGS="$CFLAGS"
fi
}}}

Unintentionally having exported but empty `CXXFLAGS` breaks '''at
least''' MPIR, because it assumes the user had intentionally set them,
so leaves them as they are (empty that is), not passing potentially
important architecture and ABI flags to the C++ compiler (e.g. `-m...`
flags to `g++`).

The solution is of course simple:  Don't (set and) export `CXXFLAGS` if
`CFLAGS` aren't exported either.  Not sure if setting `CXXFLAGS` to
`CFLAGS` in case the latter are non-empty (or empty but exported) is
desirable at all.

URL: http://trac.sagemath.org/16149
Reported by: leif
Ticket author(s): Frédéric Chapoton
Reviewer(s): Volker Braun

Author: Release Manager

src/bin/sage-env

@@ -553,7 +553,15 @@ if [ "$LDFLAGS" = "" ]; then
     LDFLAGS=""          && export LDFLAGS
 fi
 
-if [ "$CXXFLAGS" = "" ]; then
+if [ -z "$CFLAGS" ]; then
+    unset CFLAGS
+fi
+
+if [ -z "$CXXFLAGS" ]; then
+    unset CXXFLAGS
+fi
+
+if [ -n "$CFLAGS" -a -z "$CXXFLAGS" ]; then
     export CXXFLAGS="$CFLAGS"
 fi