Include lib-prefix.m4 directly and remove gettext dependency

A recent gettext release (0.25 via Homebrew) installs the M4 macros in a
different location (<prefix>/share/gettext/m4 instead of
<prefix>/share/aclocal). According to the commit messages to avoid "bad
interactions between autoreconf and autopoint".  Since we only depend
on gettext for that macro and this move makes it complicated, we can also
just integrate the macro from gnulib directly (which gettext 0.18+ relies
on anyway).

testing/scripts/build-baseimage

@@ -12,7 +12,7 @@ running_any $STRONGSWANHOSTS && die "Please stop test environment before running
 check_commands debootstrap mkfs.ext3 blockdev qemu-img qemu-nbd sfdisk
 
 # package includes/excludes
-INC=automake,autoconf,libtool,bison,flex,gperf,pkg-config,gettext,less,locales
+INC=automake,autoconf,libtool,bison,flex,gperf,pkg-config,less,locales
 INC=$INC,build-essential,libgmp-dev,libldap2-dev,libcurl4-openssl-dev,ethtool
 INC=$INC,libxml2-dev,libtspi-dev,libsqlite3-dev,openssh-server,tcpdump,psmisc
 INC=$INC,openssl,vim,sqlite3,conntrack,gdb,cmake,libltdl-dev,wget,gnupg,man-db

testing/tkm/Dockerfile

@@ -26,7 +26,7 @@
 FROM debian:bookworm
 
 ARG packages="autoconf automake bison build-essential ca-certificates ccache \
-flex gettext git gperf libssl-dev libtool pkg-config \
+flex git gperf libssl-dev libtool pkg-config \
 gnat gprbuild libahven11-dev libxmlada-schema12-dev libgmpada12-dev \
 libalog8-dev"