Changed library output name to "glfw3", and pkg-config name to libglfw3.

Since the headers are already named differently, this allows
side-by-side installation of GLFW 2.x and GLFW 3.x.

src/libglfw3.pc.in

@@ -0,0 +1,13 @@
+prefix=@CMAKE_INSTALL_PREFIX@
+exec_prefix=${prefix}
+includedir=${prefix}/include
+libdir=${exec_prefix}/lib
+
+Name: GLFW
+Description: A portable library for OpenGL, window and input
+Version: @GLFW_VERSION_FULL@
+URL: http://www.glfw.org/
+Requires.private: @GLFW_PKG_DEPS@
+Libs: -L${libdir} -lglfw3
+Libs.private: @GLFW_PKG_LIBS@
+Cflags: -I${includedir}