mkmf: fixed install directories of header files in extension
	 libraries [Bug #17761]

	When installing an extension library which provides a header, that
	header should be installed under site_ruby (or vendor_ruby when
	"--vendor" option was given to extconf.rb).  However, currently
	this file is about to be installed in the core include directory.
	---
	 lib/mkmf.rb               |  8 ++++----
	 test/mkmf/test_install.rb | 30 ++++++++++++++++++++++++++++++
	 2 files changed, 34 insertions(+), 4 deletions(-)
	 create mode 100644 test/mkmf/test_install.rb