[PATCH] (autogen.sh) improve POTFILES.in creating

[and]

• current POTFILES.in file detection can output wrong filename

xgettext output
#: src/background.c:221 src/filemanager/file.c:858 src/filemanager/file.c:952
result in
src/background.csrc/filemanager/file.csrc/filemanager/file.c

• simplify POTFILES.in file detection by using grep only
• find command search files only
• find/grep option should be Solaris OS friendly (please verify)
• move POTFILES.in creating to configure.ac (more BUILD friendly and future autogen.sh get rid possibility)
• autoconf AC_CONFIG_COMMANDS_POST support since <2.64
• comparing current vs simplify grep file detection result in additional files:

lib/search/regex.c
lib/widget/history.c
lib/widget/quick.h
src/diffviewer/internal.h
src/diffviewer/search.c
src/editor/editdraw.c
src/editor/etags.c
src/filemanager/command.c
src/vfs/tar/tar.c
src/viewer/lib.c
tests/src/executecommon.c
tests/src/executeexecute_with_vfs_arg.c

[zaytsev]

The build on Solaris seems to pass:

​https://github.com/MidnightCommander/mc/actions/runs/12491592796/job/34857805295