Make HIntLib compile with GCC 4.7
Delete all autoconf-generated files
Eliminate some Autoconf-related warnings

Autoconf still warns about HL_TEST_BUILTIN, HL_WHICH_MATH_FUN and
HL_WHICH_ABS, but I don't know how to make them "proper" since they
use variable expansion inside macros.
More fixes to also make the tests compile without -fpermissive
Add .gitignore