Revert "String literals are always const"
Revert "String literals are always const"

This reverts commit d4fe9aba3fc6f4b9cc97b100e86ffe7af5c29619.
I was confused by the fact that string literals aren't writable.
Nevertheless the type isn't const.  As extension in GCC it's const
with -Wwrite-string, which is exactly what we had before.

GCC also wonders in a comment if it's really a good idea to change
expression types based on warning flags (IMHO it's not), but let's
be compatible.  So restore the state from before.

andreiw/tinycc,avih/tinycc,mirror/tinycc,mingodad/tinycc,code4game/tinycc,avih/tinycc,code4game/tinycc,mirror/tinycc,mirror/tinycc,avih/tinycc,andreiw/tinycc,mingodad/tinycc,andreiw/tinycc,mingodad/tinycc,mingodad/tinycc,code4game/tinycc,avih/tinycc,mirror/tinycc