Equvialent to the following C code, applied to all C source files owned by
this Module:
#define name value
name and value need not live longer than the function call.
pub fn addCMacro(m: *Module, name: []const u8, value: []const u8) void
pub fn addCMacro(m: *Module, name: []const u8, value: []const u8) void {
const b = m.owner;
const graph = m.owner.graph;
const arena = graph.arena;
m.c_macros.append(arena, b.fmt("-D{s}={s}", .{ name, value })) catch @panic("OOM");
}