Android update and small improvements

-fixed code do cross compiling to android
-fixed build with GCC (workaround)
-added little benchmark
-several small fixes
-updated meson build (demos building, working with GCC, LDC and DMD)
-added some meson options
-added ImGUI bind for OpenGL3

demos/external/android/bindbc/loader/system.d

@@ -0,0 +1,55 @@
+
+//          Copyright Michael D. Parker 2018.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          http://www.boost.org/LICENSE_1_0.txt)
+
+module bindbc.loader.system;
+
+static if((void*).sizeof == 8) {
+    enum bind64 = true;
+    enum bind32 = false;
+}
+else {
+    enum bind64 = false;
+    enum bind32 = true;
+}
+
+version(Windows) enum bindWindows = true;
+else enum bindWindows = false;
+
+version(OSX) enum bindMac = true;
+else enum bindMac = false;
+
+version(linux) enum bindLinux = true;
+else enum bindLinux = false;
+
+version(Posix) enum bindPosix = true;
+else enum bindPosix = false;
+
+version(Android) enum bindAndroid = true;
+else enum bindAndroid = false;
+
+enum bindIOS = false;
+enum bindWinRT = false;
+
+version(FreeBSD) {
+    enum bindBSD = true;
+    enum bindFreeBSD = true;
+    enum bindOpenBSD = false;
+}
+else version(OpenBSD) {
+    enum bindBSD = true;
+    enum bindFreeBSD = false;
+    enum bindOpenBSD = true;
+}
+else version(BSD) {
+    enum bindBSD = true;
+    enum bindFreeBSD = false;
+    enum bindOpenBSD = false;
+}
+else {
+    enum bindBSD = false;
+    enum bindFreeBSD = false;
+    enum bindOpenBSD = false;
+}