When you have checked out a fresh copy of GMP from the repository, the
usual ./configure && make is not sufficient to build the sources. You
need to have the following extra tools installed:

* autoconf-2.65

* automake-1.11.1

* libtool-2.2.6b

Later versions of these tools may work as well. Then the following
steps are needed:

* Run the .bootstrap script, in the top source directory. This
  creates various automatically generated files such as configure and
  Makefile.in.

    ./.bootstrap

* Run the configure script, either in the source directory or in a
  separate build directory. Use the --enable-maintainer-mode option.

    ./configure --enable-maintainer-mode

Then you should be able to build the sources.