Add GLFW source tree to src/external

We need the CMake stuff for wayland configuration.
Otherwise, we would have to replicate that ourselves.
This is the full 7ef34eb06de54dd9186d3d21a401b2ef819b59e7 tree except
for tests/ examples/ and docs/

src/external/glfw/CMake/modules/FindOSMesa.cmake

@@ -0,0 +1,18 @@
+# Try to find OSMesa on a Unix system
+#
+# This will define:
+#
+#   OSMESA_LIBRARIES   - Link these to use OSMesa
+#   OSMESA_INCLUDE_DIR - Include directory for OSMesa
+#
+# Copyright (c) 2014 Brandon Schaefer <brandon.schaefer@canonical.com>
+
+if (NOT WIN32)
+
+  find_package (PkgConfig)
+  pkg_check_modules (PKG_OSMESA QUIET osmesa)
+
+  set (OSMESA_INCLUDE_DIR ${PKG_OSMESA_INCLUDE_DIRS})
+  set (OSMESA_LIBRARIES   ${PKG_OSMESA_LIBRARIES})
+
+endif ()