Fix autogen.sh failing to find gettext

[sleeptightAnsiC]

...which was preventing proper ./configure generation and making build from git sources impossible. Now it should look for gettext inside of ./m4/* before trying to get it from system directories.

Running autogen.sh still produces some warnings about missing gettext variables and errors about missing 'Makefile.am' files but './configure && make' should succeed anyway without any issues.

This issue was mentioned in:
#659 (comment) #658 (comment)

Tested on latest ArchLinux with:
gettext (GNU gettext-runtime) 0.26
automake (GNU automake) 1.18.1
autoconf (GNU Autoconf) 2.72

also tested on Ubuntu 25.04 with:
gettext (GNU gettext-runtime) 0.23.1
automake (GNU automake) 1.17
autoconf (GNU Autoconf) 2.72

[sleeptightAnsiC]

Also few notes after spending some time on figuring out how the build process of JWM works:

• JWM itself does not seem to need Automake, only gettext really needs this, so I would suggest to either (a) fully commit to Automake and turn every Makefile.in into corresponding Makefile.am or (b) try to remove Automake macros from configure.ac and do this stuff manually.
• Current ./autogen.sh runs aclocal and automake and only then autoreconf, but if I understand right, autoreconf is able to run those things (and much more) on its own.
• Notice that errors produced by Automake (about missing Makefile.am) are fatal but since autogen.sh does not use set -e, the script ignores any failures of sub commands and continues.
• I'm afraid that this setup is simply wrong and just happens to work right now, and will most likely break again.

That said, I'm not very familiar with Autotools so maybe I'm just spreading nonsense right now... (sorry)

[sleeptightAnsiC]

Also it seem that people at AUR had this exact issue according to: https://aur.archlinux.org/packages/jwm-git#comment-1038518

[sleeptightAnsiC]

Rethinking this problem after some time... I think this can be also fixed by adding ACLOCAL_AMFLAGS = -I m4 somewhere at the very top of configure.ac (but I haven't tested it)