Add --lib-only configure option

This is a short hand for --disable-app --disable-examples
--disable-hpack-tools --disable-python-bindings, for users who want to
build libnghttp2 only.

configure.ac

@@ -104,6 +104,11 @@ AC_ARG_ENABLE([failmalloc],
                     [Do not build failmalloc test program])],
     [request_failmalloc=$enableval], [request_failmalloc=yes])
 
+AC_ARG_ENABLE([lib-only],
+    [AS_HELP_STRING([--enable-lib-only],
+                    [Build libnghttp2 only.  This is a short hand for --disable-app --disable-examples --disable-hpack-tools --disable-python-bindings])],
+    [request_lib_only=$enableval], [request_lib_only=no])
+
 AC_ARG_WITH([libxml2],
     [AS_HELP_STRING([--with-libxml2],
                     [Use libxml2 [default=check]])],
@@ -150,6 +155,13 @@ PKG_PROG_PKG_CONFIG([0.20])
 
 AM_PATH_PYTHON([2.7],, [:])
 
+if [test "x$request_lib_only" = "xyes"]; then
+  request_app=no
+  request_hpack_tools=no
+  request_examples=no
+  request_python_bindings=no
+fi
+
 if [test "x$request_python_bindings" != "xno"]; then
   AX_PYTHON_DEVEL([>= '2.7'])
 fi