When you start from the actual repository source and not a distribution you can use
make -f Makefile.build
to run the necessary libtool, autoconf, etc routines and copy a few
additional M4 scripts to the right place to put the directory in a
similar state to that of a distribution directory. Then you can run
the usual ./configure; make; sudo make install.
If the state of the directory is messed up, then you can run
make -f Makefile.build clean
to do a brutal cleanup of everything. Yes, there are various clean
commands in Makefile, but this seems more robust and convenient when
developing. After this command git status should not list any magic
files that you did not write. Finally, you can use
make -f Makefile.build dist
to build a distribution vec-<version>.tar.gz in a single
command. This merely sets up things and then runs the usual make dist.