Expose public garbage collector API for C language

You can now do epic things like this:

    puts(_gc(xasprintf("%d", 123)));

The _gc() API is shorthand for _defer() which works like Go's keyword:

    const char *s = xasprintf("%d", 123);
    _defer(free, s);
    puts(s);

Be sure to always use -fno-omit-frame-pointer which makes code fast too.

Enjoy! See also #114

build/config.mk

@@ -282,11 +282,6 @@ endif
 # such as MSVC or XCode. You can run your binary objects through a tool
 # like objconv to convert them to COFF or MachO. Then use ANSI mode to
 # rollup one header file that'll enable linkage with minimal issues.
-#
-# Lastly note that in some cases, such as gc(), there simply isn't any
-# ANSI workaround available. It's only in cases like that when we'll use
-# the __asm__() header workaround, rather than simply removing it. We do
-# however try to do that much less often than mainstream C libraries.
 
 ifeq ($(MODE), ansi)