pull in and build glcxx submodule

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "libs/googletest"]
 	path = libs/googletest
 	url = git://github.com/google/googletest
+[submodule "libs/glcxx"]
+	path = libs/glcxx
+	url = git://github.com/holtrop/glcxx.git

wscript

@@ -11,10 +11,12 @@ def configure(conf):
 
 def build(bld):
     includes = [p for p in bld.path.ant_glob("src/**", dir = True) if os.path.isdir(p.abspath())]
+    includes += ["libs/glcxx/include"]
     bld(features = "c cprogram cxx cxxprogram",
         target = "jes",
-        source = bld.path.ant_glob(["src/**/*.cc", "src/**/*.c"]),
+        source = bld.path.ant_glob(["src/**/*.cc", "src/**/*.c", "libs/glcxx/src/glcxx/*"]),
         includes = includes,
+        defines = ["GLCXX_GL_INCLUDE=\"gl3w.h\""],
         cxxflags = ["-Wall", "-std=gnu++14"],
         lib = ["dl", "GL"])