autotools build system should create pkgconfig file

[mkoeppe]

(The cmake build system creates one.)

Here are some resources:

• https://tecnocode.co.uk/2014/12/09/a-checklist-for-writing-pkg-config-files/
• https://lists.gnu.org/archive/html/automake/2008-05/msg00060.html
• https://autotools.io/pkgconfig/pkg_check_modules.html
• http://stackoverflow.com/questions/2164089/using-pkg-config-with-autotools
• http://stackoverflow.com/questions/10220946/pkg-check-modules-considered-harmful/10229811#10229811
• http://stackoverflow.com/questions/8578181/using-the-pkg-config-macro-pkg-check-modules-failing
• http://tirania.org/blog/archive/2012/Oct-20.html

[fingolfin]

It would be really useful to have this, to make using libnormaliz a bit easier; in particular for determining the correct CPPFLAGS/CXXFLAGS/LDFLAGS (to enable e.g. C++14 or C++18 or whatever; and for the required compiler/linker flags for dependencies of libnormaliz, such as GMP, (e)antic, etc.

@dimpase added this for GAP / libgap in gap-system/gap#5080 and it doesn't require that much code.

Unfortunately I really don't have time right now, otherwise I'd look into it myself, but @w-bruns let's keep it in mind for when we next meet to work on Normaliz.jl :-)