autotools: make windows build work also, I've added back the method I made for getting the vars as variables rather than separate files, so less clutter in tmp folders :)

lib_LTLIBRARIES = libpugixml.la
libpugixml_la_SOURCES = src/pugixml.cpp
include_HEADERS = src/pugixml.hpp

libpugixml_la_CXXFLAGS = -std=c++11
ACLOCAL_AMFLAGS = -I m4