Previously, Graphene supported Glibc version 2.19 only. This commit
adds support for Glibc v2.23 (default in Ubuntu 16.04) and Glibc
v2.27 (default in Ubuntu 18.04). The exact version can be chosen by
specifying GLIBC_VERSION during Graphene build. The default version
to be built is v2.27.
Note that for the best GDB experience it is advised to build Graphene
with the same Glibc version as installed on the host OS.
