meson: add 'gpl' option and only build plugins with (A)GPL deps if explicitly enabled

Require explicit opt-in to build plugins with (A)GPL dependencies.
31 jobs for !1084 with gpl-opt-in in 2 minutes and 19 seconds (queued for 1 second)
detached
Status Name Job ID Coverage
  Trigger
manual trigger #14746138
manual
 
  Build Docker
created fedora amd64 docker #14746139
passed gst-indent amd64 docker #14746140

00:01:26

created windows amd64 docker #14746141
windows shell 1809
 
  Preparation
passed gst indent #14746142

00:00:53

 
  Build
created build clang fedora x86_64 #14746146
created build documentation #14746150
created build msys2 #14746149
docker windows 1809 allowed to fail manual
created build nodebug fedora x86_64 #14746143
created build static fedora x86_64 #14746144
created build static nodebug fedora x86_64 #14746145
created build vs2017 amd64 #14746147
docker windows 1809
created build vs2017 x86 #14746148
docker windows 1809
created cerbero trigger #14746151
 
  Test
created check fedora #14746152
gstreamer
created integration testsuites fedora 1/4 #14746153
gstreamer
created integration testsuites fedora 2/4 #14746154
gstreamer
created integration testsuites fedora 3/4 #14746155
gstreamer
created integration testsuites fedora 4/4 #14746156
gstreamer
created valgrind bad #14746165
gstreamer
created valgrind base 1/4 #14746159
gstreamer
created valgrind base 2/4 #14746160
gstreamer
created valgrind base 3/4 #14746161
gstreamer
created valgrind base 4/4 #14746162
gstreamer
created valgrind core 1/2 #14746157
gstreamer
created valgrind core 2/2 #14746158
gstreamer
created valgrind ges #14746166
gstreamer
created valgrind good #14746163
gstreamer
created valgrind ugly #14746164
gstreamer
 
  Integrate
created gstreamer-full #14746167
created gstreamer-full-minimal #14746168