xfce4-dev-tools can't be installed in /usr/local anymore

Related to commit 0e2a8cb7

On a fresh instance, grab xfce4-dev-tools master, ./autogen.sh && make install: it will install m4 files to /usr/local/share.

Then take libxfce4util, ./autogen.sh fails, it can't find XDT_* definitions because they are installed in /usr/local, but aclocal search in /usr (system) prefix.