gc: enable on macos and add leak detection fns

vlib/builtin/builtin_d_gcboehm.v

@@ -9,10 +9,16 @@ $if windows {
 
 #include <gc.h>
 
-#flag -lgc
+#flag linux -lgc
+#flag darwin @VROOT/thirdparty/bdwgc/extra/.libs/gc.o
 
 fn C.GC_MALLOC(n size_t) voidptr
 
 fn C.GC_REALLOC(ptr voidptr, n size_t) voidptr
 
 fn C.GC_FREE(ptr voidptr)
+
+fn C.GC_set_find_leak(int)
+
+// fn C.CHECK_LEAKS()
+fn C.GC_gcollect()

vlib/v/gen/c/cgen.v

@@ -5168,7 +5168,7 @@ fn (mut g Gen) write_init_function() {
 		// g.writeln('g_str_buf = malloc( ${mb_size} * 1024 * 1000 );')
 	}
 	if g.pref.prealloc {
-		g.writeln('g_m2_buf = malloc(50 * 1000 * 1000);')
+		g.writeln('g_m2_buf = malloc(150 * 1000 * 1000);')
 		g.writeln('g_m2_ptr = g_m2_buf;')
 	}
 	// NB: the as_cast table should be *before* the other constant initialize calls,