Automatically disable airbag for mingw builds

Bug #362128 r=bsmedberg

configure

@@ -15414,7 +15414,7 @@ fi
 fi
 
 
-if test "$OS_ARCH" = "WINNT"; then
+if test "$OS_ARCH" = "WINNT" -a -z "$GNU_CC"; then
    MOZ_AIRBAG=1
 fi
 

configure.in

@@ -5517,7 +5517,7 @@ dnl ========================================================
 dnl = Airbag crash reporting (on by default on supported platforms)
 dnl ========================================================
 
-if test "$OS_ARCH" = "WINNT"; then
+if test "$OS_ARCH" = "WINNT" -a -z "$GNU_CC"; then
    MOZ_AIRBAG=1
 fi