mesa driconf: Add macro to specify an option with a quoted default value.

mesa driconf: Add macro to specify an option with a quoted default value.

The default values true and false will expand to "1" and "0" when
gcc -std=c99, causing bool option defaults to generate runtime failures.

One solution is to specify bool option defaults quoted as "true" and "false".
Add a macro to assist this.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>