document DESTDIR instead of broken DESTROOT

Streamline the Windows/mingw build rule to simplify in-tree builds where pd.dll may be in a different subdirectory of .

nw.js (Pd-l2ork 2.x) support: opening the Lua script file in Purr Data now does the right thing on Windows, too

macOS compilation: Pd-extended is getting a bit long in the tooth, let's assume a vanilla install instead

Upstream version 0.7.3

# gpg: Signature made Sun 24 Jan 2016 09:49:54 PM CET using RSA key ID 7F7A36F8
# gpg: Good signature from "IOhannes m zmölnig <zmoelnig@umlaeute.mur.at>" [ultimate]
# gpg:                 aka "IOhannes m zmölnig <zmoelnig@iem.at>" [ultimate]
# gpg:                 aka "[jpeg image of size 5377]" [ultimate]
# gpg:                 aka "IOhannes m zmölnig (IEM - Network Operation Center) <noc@iem.at>" [ultimate]
# gpg:                 aka "IOhannes m zmölnig (mur.at - Verein zur Förderung von Netzwerkkunst) <zmoelnig@mur.at>" [ultimate]
# gpg:                 aka "IOhannes m zmölnig (forum::für::umläute - Network Operation Center) <noc@umlaeute.mur.at>" [ultimate]
# gpg:                 aka "Johannes Zmölnig (University of Music and Performing Arts - Graz) <johannes.zmoelnig@kug.ac.at>" [ultimate]
# gpg:                 aka "IOhannes m zmölnig (forum::für::umläute) <forum@umlaeute.mur.at>" [ultimate]
# gpg:                 aka "IOhannes m zmölnig (Debian/GNU) <umlaeute@debian.org>" [ultimate]

the secondary dependency on (virtual) 'pd'
can be satisfied by 'puredata-core'

no change required

run the script in a freshly checked out repository for fine-tuning

use https://.../cgit/...

since we override CFLAGS completely, we also need to
pass "-fPIC"

for helping