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).

scripts/test.sh

@@ -401,10 +401,9 @@ macos)
 			--enable-socket-default --enable-sshkey --enable-stroke
 			--enable-swanctl --enable-unity --enable-updown
 			--enable-x509 --enable-xauth-generic"
-	DEPS="automake autoconf libtool bison gettext gperf pkgconf [email protected] curl"
+	DEPS="automake autoconf libtool bison gperf pkgconf [email protected] curl"
 	BREW_PREFIX=$(brew --prefix)
 	export PATH=$BREW_PREFIX/opt/bison/bin:$PATH
-	export ACLOCAL_PATH=$BREW_PREFIX/opt/gettext/share/aclocal:$ACLOCAL_PATH
 	for pkg in [email protected] curl
 	do
 		PKG_CONFIG_PATH=$BREW_PREFIX/opt/$pkg/lib/pkgconfig:$PKG_CONFIG_PATH
@@ -485,11 +484,11 @@ deps)
 	case "$OS_NAME" in
 	linux)
 		sudo apt-get update -y && \
-		sudo apt-get install -y automake autoconf libtool pkgconf bison flex gperf gettext $DEPS
+		sudo apt-get install -y automake autoconf libtool pkgconf bison flex gperf $DEPS
 		;;
 	alpine)
 		apk add --no-cache build-base automake autoconf libtool pkgconfig && \
-		apk add --no-cache bison flex gperf gettext-dev tzdata $DEPS
+		apk add --no-cache bison flex gperf tzdata $DEPS
 		;;
 	macos)
 		brew update && \
@@ -497,7 +496,7 @@ deps)
 		;;
 	freebsd)
 		pkg install -y automake autoconf libtool pkgconf && \
-		pkg install -y bison flex gperf gettext $DEPS
+		pkg install -y bison flex gperf $DEPS
 		;;
 	esac
 	exit $?