fix default inc/lib path detection on msys

using the Makefile target separator to determine the host system is of
course not a terribly good idea. the correct variable to query would be
DIR_SEPARATOR, which always reflects the host. however, a direct os
query is less obscure in this case.

Task-number: QTBUG-54346
Change-Id: I6bc1372a7c9a84a241a97f9f767f046b026411c3
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

mkspecs/features/default_pre.prf

@@ -30,7 +30,7 @@ isEmpty(QMAKE_DEFAULT_INCDIRS):!host_build {
     # Get default include and library paths from compiler
     #
     gcc {
-        equals(QMAKE_DIR_SEP, /) {
+        !equals(QMAKE_HOST.os, Windows) {
             cmd_prefix = "LC_ALL=C"
             cmd_suffix = "</dev/null >/dev/null"
         } else {