universal-ctags: Fix build for Linux

On Linux (reproducible in the homebrew/brew Docker container) this was failing to build due to the following error:

==> Installing universal-ctags/universal-ctags/universal-ctags dependency: docutils
==> Pouring docutils-0.16_2.x86_64_linux.bottle.tar.gz
    /home/linuxbrew/.linuxbrew/Cellar/docutils/0.16_2: 790 files, 8.6MB
==> Installing universal-ctags/universal-ctags/universal-ctags --HEAD
==> ./autogen.sh
==> ./configure --prefix=/home/linuxbrew/.linuxbrew/Cellar/universal-ctags/HEAD-62f0144
==> make
Last 15 lines from /root/.cache/Homebrew/Logs/universal-ctags/03.make:
  CC       main/libctags_a-debug.o
  CC       main/readtags-mio.o
  CC       main/mini_geany-mini-geany.o
RST2MAN    man/ctags.1
RST2MAN    man/readtags.1
/home/linuxbrew/.linuxbrew/Cellar/docutils/0.16_2/libexec/bin/python3.8: error while loading shared libraries: libpython3.8.so.1.0: cannot open shared object file: No such file or directory
RST2MAN    man/tags.5
make[1]: *** [Makefile:5960: man/readtags.1] Error 127
make[1]: *** Waiting for unfinished jobs....
/home/linuxbrew/.linuxbrew/Cellar/docutils/0.16_2/libexec/bin/python3.8: error while loading shared libraries: libpython3.8.so.1.0: cannot open shared object file: No such file or directory
make[1]: *** [Makefile:5960: man/ctags.1] Error 127
/home/linuxbrew/.linuxbrew/Cellar/docutils/0.16_2/libexec/bin/python3.8: error while loading shared libraries: libpython3.8.so.1.0: cannot open shared object file: No such file or directory
make[1]: *** [Makefile:5962: man/tags.5] Error 127
make[1]: Leaving directory '/tmp/universal-ctags-20200628-19191-titkke'
make: *** [Makefile:1304: all] Error 2

This should avoid --without-docutils becoming a common flag that users need to specify.

Thanks for maintaining this formula and universal-ctags as software!

universal-ctags / homebrew-universal-ctags

universal-ctags: Fix build for Linux #52