Add -std=c++14 to build_cross_tools_gcc4

CLang fails to build ISL otherwise, and if you are setting up
cross compiling, you should have a modern enough compiler

Fixes #16434

Change-Id: Ieb4a33015899896ea3bb2d1abc59b10fe0b4b3f2
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3730
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>

build/scripts/build_cross_tools_gcc4

@@ -25,7 +25,8 @@ additionalMakeArgs=$*
 gdbSourceDir="$HAIKU_USE_GDB"
 
 ccFlags="-O2"
-cxxFlags="-O2"
+#CLang needs c++14 to build ISL: https://dev.haiku-os.org/ticket/16434
+cxxFlags="-O2 -std=c++14"
 binutilsTargets="$haikuMachine"
 gdbTarget="$haikuMachine"
 case $haikuMachine in