add macos workflow, fix filesystem_async max thread count

scripts/build-macos-amd64.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+git submodule init && git submodule update
+
+brew install sdl2
+
+./waf configure -T debug --64bits --disable-warns $* &&
+./waf build

scripts/tests-macos-amd64.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+git submodule init && git submodule update
+./waf configure -T release --sanitize=address,undefined --disable-warns --tests -8 --prefix=out/ $* &&
+./waf install &&
+cd out &&
+DYLD_LIBRARY_PATH=bin/ ./unittest || exit 1