use a macro to set GUIDIR

This makes it easier to point the gui dir at a local copy of the repo for
easier development. You can use this to make it easier to test out changes
to the GUI in purr-data/pd/nw.

Setting GUIDIR should even work if you haven't installed Purr Data to the
system. Just make sure the nw.js binary has been downloaded and expanded to
purr-data/pd/nw/nw. (This happens automatically when you build.)
10 jobs for define-custom-gui-path in 381 minutes and 54 seconds (queued for 26 minutes and 40 seconds)
Status Job ID Name Coverage
  Test
passed #9792
i386 jessie debian
debian_jessie_i386_deb

00:28:24

passed #9793
x86_64 jessie debian
debian_jessie_x86_64_deb

00:33:34

passed #9794
x86_64 stretch debian
debian_stretch_x86_64_deb

00:31:48

passed #9798
dmg v10.11 osx
osx_10.11_x86_64_dmg

00:18:35

passed #9799
dmg v10.8 osx
osx_10.8_x86_64_dmg

00:18:36

passed #9801
stretch arm raspbian
raspbian_stretch_armv7l_deb

00:57:48

passed #9795
i386 v14.04 ubuntu
ubuntu_14.04_i386_deb

00:37:08

passed #9796
x86_64 v14.04 ubuntu
ubuntu_14.04_x86_64_deb

00:32:59

passed #9797
x86_64 v16.04 ubuntu
ubuntu_16.04_x86_64_deb

00:40:05

passed #9800
i386 windows
windows_i386_innosetup

01:22:53