build: use install-info when installing mu4e/guile

So the docs can be found.

build-aux/meson-install-info.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+infodir=$1
+infofile=$2
+
+# Meson post-install script to update info metadata
+install-info --info-dir ${MESON_INSTALL_DESTDIR_PREFIX}/${infodir} \
+	     ${MESON_INSTALL_DESTDIR_PREFIX}/${infodir}/${infofile}
+
+gzip --force ${MESON_INSTALL_DESTDIR_PREFIX}/${infodir}/${infofile}