Add Premake wrapper script for usage in CI.

build/premake.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+
+DIR=$( cd "$( dirname "$0" )" && pwd )
+
+case "$(uname -s)" in
+
+   Darwin)
+     $DIR/premake5-osx $*
+     ;;
+
+   Linux)
+     $DIR/premake5-linux $*
+     ;;
+
+   CYGWIN*|MINGW32*|MSYS*|MINGW*)
+     $DIR/premake5.exe $*
+     ;;
+
+   *)
+    echo "Unsupported platform"
+    exit 1
+     ;;
+esac