Add naive make install for Linux

This enables a `sudo make install` that copies the build directory to
the PREFIX directory (/opt/OpenVR-AdvancedSettings/AdvancedSettings by
default).

The entire directory is being copied because Qt is incredibly weird
about how the INSTALLS variable works. From testing it appears that only
files that exist before the build process has started (even the
file_copy process) will be eligible for installation.

If the /OpenVR-AdvancedSettings/ directory is excluded, Qt will attempt
to remove the /opt/ folder on `make uninstall`, this is obviously not
desirable.

advancedSettings.pro

@@ -94,3 +94,16 @@ win32 {
     # for running executables.
     QMAKE_POST_LINK = cmd /c $$WINDEPLOYQT_FULL_LINE
 }
+
+# Add make install support
+unix {
+    isEmpty(PREFIX){
+        PREFIX = /opt/OpenVR-AdvancedSettings
+    }
+
+    application.path = $$PREFIX
+    application.files = $$COPY_DEST_DIR
+
+    INSTALLS += application
+}
+