glcpp: Fix handling of "#line 0"

glcpp: Fix handling of "#line 0"

The existing DECIMAL_INTEGER pattern is the correct thing to use when
looking for a C decimal integer, (that is, a digit-sequence not
starting with 0 which would instead be an octal integer).

But for #line, we really want to accept any digit sequence, (including
"0"), and always interpret it as a decimal constant. So we add a new
DIGITS pattern for this case.

This should fix the compilation failure noted in bug #28138

	https://bugs.freedesktop.org/show_bug.cgi?id=28138

(Though the generated file will not be updated until the next commit.)