The format conversion from nroff to rst is done by @ffes.
I take over the work.

Changes done by Universal-ctags project have been documented in
a file named "hacking guide" with rst format. The man page (ctags.1)
has not been updated well. The man page is written in nroff format.

Using different formats in the documents makes hard to share contents
between two files.

This commit utilizes rst2man, a tool for converting the format of
input files from rst to nroff.  It means contents can share between
two documents.

Here after man page should be written rst format (ctags.1.rst.in).

Using AC_PATH_PROG is suggested by @codebrainz.
Availability of rst2man.py on Cygwin is reported by @ffes.
@b4n simplfies the manual page generation process(Don't run rst2man in
the configure process).