Dear Guix,

The following patch adds abseil-cpp.  I want to add it because it seems
to be a dependency of tensorflow-1.15.0 (which I am trying to update
to).

There are two not-so-perfect things with the patch:
1. I had to use the latest Git commit because it adds bits to build
shared libraries.
2. I disabled the tests because it attempts to download "googletest". 
I tried modifying the build system to use "googletest" from Guix but
that wasn't succesful.

Nevertheless, I hope this patch is acceptable.

Kind regards,
Roel Janssen