Fix-ups for macOS (#291)

* Do not use precomp headers target with GCC on Apple

* CMakeLists: also link to AudioUnit on Apple

* platform.cpp: add missing <unistd.h>

---------

Co-authored-by: ouwou <26526779+ouwou@users.noreply.github.com>

src/platform.cpp

@@ -4,6 +4,10 @@
 #include <fstream>
 #include <string>
 
+#ifdef __APPLE__
+    #include <unistd.h>
+#endif
+
 #include <spdlog/spdlog.h>
 
 using namespace std::literals::string_literals;