Add check for cython and substitute python and cython in Makefile.am

configure.ac

@@ -73,6 +73,9 @@ AC_ARG_WITH([libxml2],
                     [disable support for libxml2])],
     [], [with_libxml2=yes])
 
+dnl Define variables
+AC_ARG_VAR([CYTHON], [the Cython executable])
+
 dnl Checks for programs
 AC_PROG_CC
 AC_PROG_CXX
@@ -82,6 +85,7 @@ AC_PROG_MAKE_SET
 AM_PROG_CC_C_O
 PKG_PROG_PKG_CONFIG([0.20])
 AM_PATH_PYTHON([2.6],, [:])
+AC_CHECK_PROGS([CYTHON], [cython.py cython])
 
 AX_CXX_COMPILE_STDCXX_11([noext], [optional])
 
@@ -354,4 +358,6 @@ AC_MSG_NOTICE([summary of build options:
     Examples:       ${enable_examples}
     Hdtest:         ${enable_hdtest}
     Failmalloc:     ${request_failmalloc}
+    Python:         ${PYTHON} ${PYTHON_VERSION}
+    Cython:         ${CYTHON}
 ])

python/Makefile.am

@@ -25,12 +25,18 @@ EXTRA_DIST = README.rst cnghttp2.pxd nghttp2.pyx setup.py
 
 PYSETUP_INCLUDE_DIRS=$(top_srcdir)/lib/includes:$(top_srcdir)/lib
 PYSETUP_LIBDIRS=$(top_builddir)/lib/.libs
+CYTHON = @CYTHON@
+PYTHON = @PYTHON@
+
+.PHONY: help build_ext
+
+help:
+	@echo "Please use \`make <target>\` where <target> is one of"
+	@echo "  build_ext  to build Python @PYTHON_VERSION@ nghttp2 extension"
 
 nghttp2.c: nghttp2.pyx cnghttp2.pxd
-	cython nghttp2.pyx
-
-.PHONY: build_ext
+	$(CYTHON) nghttp2.pyx
 
 build_ext: nghttp2.c
-	python setup.py build_ext --include-dirs=$(PYSETUP_INCLUDE_DIRS) \
+	$(PYTHON) setup.py build_ext --include-dirs=$(PYSETUP_INCLUDE_DIRS) \
 	    --library-dirs=$(PYSETUP_LIBDIRS)