Use proper cmake detection of features.

This attempts to organize all needed items and places appropriate defines
in a single location. Redundant comparisons/defines were removed when noticed.

A few caveats, however:

* This may be better targeted towards the 5_1_0 branch instead of master right now.
* Cmake will run a little slower on configuration/generation now. That's due to the sanity checks it runs.
* There are some more checks to be added later, but this should be a solid start.

Thanks to the [libical](https://github.com/libical/libical) project for inspiration.

src/arch/InputHandler/InputHandler_Linux_Event.cpp

@@ -5,8 +5,13 @@
 #include "LinuxInputManager.h"
 #include "GamePreferences.h" //needed for Axis Fix
 
+#if defined(HAVE_UNISTD_H)
 #include <unistd.h>
+#endif
+
+#if defined(HAVE_FCNTL_H)
 #include <fcntl.h>
+#endif
 
 #include <errno.h>
 #include <sys/types.h>

src/arch/InputHandler/InputHandler_Linux_Joystick.cpp

@@ -5,8 +5,12 @@
 #include "LinuxInputManager.h"
 #include "RageInputDevice.h" // NUM_JOYSTICKS
 
+#if defined(HAVE_UNISTD_H)
 #include <unistd.h>
+#endif
+#if defined(HAVE_FCNTL_H)
 #include <fcntl.h>
+#endif
 
 #include <errno.h>
 #include <sys/types.h>

src/arch/InputHandler/InputHandler_Linux_PIUIO.cpp

@@ -5,8 +5,12 @@
 
 #include <stdio.h>
 #include <string.h>
+#if defined(HAVE_UNISTD_H)
 #include <unistd.h>
+#endif
+#if defined(HAVE_FCNTL_H)
 #include <fcntl.h>
+#endif
 
 #include <errno.h>
 #include <sys/types.h>

src/arch/InputHandler/InputHandler_Linux_tty.cpp

@@ -16,7 +16,9 @@
 #include <errno.h>
 
 #include <sys/ioctl.h>
+#if defined(HAVE_FCNTL_H)
 #include <fcntl.h>
+#endif
 
 #include <linux/kd.h>
 #include <linux/keyboard.h>

src/arch/InputHandler/LinuxInputManager.cpp

@@ -8,7 +8,11 @@
 #include "Foreach.h"
 
 #include <string> // std::string::npos
+
+#if defined(HAVE_DIRENT_H)
 #include <dirent.h>
+#endif
+
 #include <errno.h>
 
 RString getDevice(RString inputDir, RString type)