depends: allow CC/CXX to be overridden during configure

depends/config.site.in

@@ -61,9 +61,12 @@ fi
 CPPFLAGS="-I$depends_prefix/include/ $CPPFLAGS"
 LDFLAGS="-L$depends_prefix/lib $LDFLAGS"
 
-CC="@CC@"
-CXX="@CXX@"
-OBJC="${CC}"
+if test -n "@CC@" -a -z "${CC}"; then
+  CC="@CC@"
+fi
+if test -n "@CXX@" -a -z "${CXX}"; then
+  CXX="@CXX@"
+fi
 PYTHONPATH=$depends_prefix/native/lib/python/dist-packages:$PYTHONPATH
 
 if test -n "@AR@"; then