meson: Pass -DGST_STATIC_COMPILATION for static builds

This is only needed on Windows when building with MSVC, but it is safe
to pass it everywhere.

Closes gstreamer/gstreamer#398
18 jobs for fix-static-builds in 45 minutes and 45 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #294398
gst indent

00:00:29

passed #294397
manifest

00:00:39

 
  Build
passed #294399
build fedora x86_64

00:05:25

passed #294400
build nodebug fedora x86_64

00:05:25

 
  Test
passed #294401
check fedora

00:03:10

passed #294402
integration testsuites fedora

00:07:43

 
  Full Builds
passed #294404
build cerbero android universal

00:11:11

passed #294405
build cerbero cross win32

00:07:23

passed #294406
build cerbero cross win64

00:05:25

passed #294403
build cerbero fedora x86_64

00:06:11

 
  Full Tests
passed #294411
valgrind bad

00:07:17

passed #294408
valgrind base

00:08:39

passed #294407
valgrind core

00:09:58

passed #294792
valgrind ges

00:03:25

passed #294409
valgrind good

00:07:47

passed #294410
valgrind ugly

00:02:06

failed #294412
valgrind ges

00:09:25

 
  Apps
passed #294413
android universal examples

00:07:20