M4 macros in the 3rd-party aclocal directory are not found

I have locally installed a newer version of automake (1.15) on a system which I do not have root access to. The local directory's bin folder is the first directory in my PATH.

I have also locally installed gtk+-2.0, and I now want to build and install an application which requires gtk as a dependency, i.e. the configure.ac script for my desired application contains:

PKG_CHECK_MODULES(GTK, [gtk+-2.0 >= 2.0])

To ensure the m4 macros are found, I copied all of the .m4 files in gtk_install_dir/lib/pkgconfig/ into the location that aclocal --print specifies (automake_install_dir/share/aclocal/).

I also set PKG_CONFIG_PATH=gtk_install_dir/lib/pkgconfig.

However, when I run:

$ libtoolize
$ aclocal
$ autoconf
$ automake
$ ./configure

I get an error message:

./configure: line 13106: syntax error near unexpected token `GTK,'
./configure: line 13106: `PKG_CHECK_MODULES(GTK, gtk+-2.0 >= 2.0)'

Running aclocal --verbose I can clearly see that there is no mention of gtk, so it is clearly not finding the gtk+ macros.

I have not used these build tools before, am I missing something? I seemingly can't get the configure script to recognise that gtk is installed!

Any advice is much appreciated, thanks