Builtin definitions (and functions with a body) should always be const unless prefixed by var

[divVerent]

gmqcc permits this code, whereas fteqcc does not:

void() foo = #1;

void() bar = {
	foo = bar;
};

fteqcc's error:

foo.qc:4: warning Q106: Assignment to const
foo.qc:1:    const void() foo is defined here
foo.qc:4: warning Q106: Assignment to constant foo
foo.qc:1:    const void() foo is defined here

OTOH gmqcc refuses this code, whereas fteqcc allows it:

void() foo_builtin = #1;
var void() foo = foo_builtin;

void() bar = {
	foo = bar;
};

gmqcc's error:

foo.qc:2:24: error: initializer is non constant

I think the problem is that builtin definitions aren't const in gmqcc, but should be. The issue reproduces equally if the functions have a body instead of a builtin, and also equally when using C-style function declarations, and regardless of -std=.

[divVerent]

Filed https://gitlab.com/xonotic/xonotic-data.pk3dir/-/merge_requests/1380 for Xonotic to decide how to prepare for fixing this bug.