build: add conditional autoreconf

[dabrain34]

No description provided.

[g-maxime]

I have nothing against the changes in autogen.sh itself.
But autotools aren't supposed to be run by the final user.

Extra, this fail on our macOS build environment:
configure.ac:29: error: possibly undefined macro: AC_PROG_LIBTOOL If this token and others are legitimate, please use m4_pattern_allow. See the Autoconf documentation. autoreconf: /opt/local/bin/autoconf failed with exit status: 1

[JeromeMartinez]

I am reluctant to have default behavior to launch ./configure, it is not the previous behavior and autogen.sh is planned for generating files for the final user, here we would have to change our build scripts in other places if we don't want to launch unnecessarily ./configure.

[JeromeMartinez]

But autotools aren't supposed to be run by the final user.

I am fine with that if autotools are installed, especially if it prevents the user to launch our scripts (I understand from the issue ticket that they fail if autotools is installed).

Extra, this fail on our macOS build environment

Maybe a warning already existing due to the bad configure.ac, and failing due to added "--warnings=all"? Maybe good to fix this warning at the same time.