build: Fix build on FreeBSD

- ftruncate64() does not exist on FreeBSD
- add missing environ(7) declaration

common/Linux/LnxHostSys.cpp

@@ -174,7 +174,7 @@ void* HostSys::CreateSharedMemory(const char* name, size_t size)
 	shm_unlink(name);
 
 	// ensure it's the correct size
-#ifndef __APPLE__
+#if !defined(__APPLE__) && !defined(__FreeBSD__)
 	if (ftruncate64(fd, static_cast<off64_t>(size)) < 0)
 #else
 	if (ftruncate(fd, static_cast<off_t>(size)) < 0)

common/Linux/LnxMisc.cpp

@@ -72,6 +72,8 @@ std::string GetOSVersionString()
 
 static bool SetScreensaverInhibitX11(const WindowInfo& wi, bool inhibit)
 {
+	extern char **environ;
+
 	const char* command = "xdg-screensaver";
 	const char* operation = inhibit ? "suspend" : "resume";
 	std::string id = fmt::format("0x{:X}", static_cast<u64>(reinterpret_cast<uintptr_t>(wi.window_handle)));