meson: build gir even when cross-compiling if introspection was enabled explicitly

This can be made to work in certain circumstances when
cross-compiling, so default to not building g-i stuff
when cross-compiling, but allow it if introspection was
enabled explicitly via -Dintrospection=enabled.

See gstreamer/gstreamer#454 and gstreamer/gstreamer#381.
27 jobs for introspection-cross-compile-base in 65 minutes and 6 seconds (queued for 20 seconds)
Status Job ID Name Coverage
  Preparation
passed #769063
gst indent

00:00:30

passed #769062
manifest

00:00:35

 
  Build
passed #769073
build cerbero cross-android universal

00:14:35

manual #769077
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #769074
build cerbero cross win32

00:07:55

passed #769075
build cerbero cross win64

00:06:24

passed #769072
build cerbero fedora x86_64

00:07:57

passed #769076
gst-macos-10.14
build cerbero macos x86_64

00:06:49

passed #769068
build clang fedora x86_64

00:06:38

passed #769064
build fedora x86_64

00:07:00

passed #769071
docker windows 1607
build msys2

00:14:31

passed #769065
build nodebug fedora x86_64

00:02:40

passed #769066
build static fedora x86_64

00:13:29

passed #769067
build static nodebug fedora x86_64

00:05:13

passed #769069
docker windows 1607
build vs2017 amd64

00:11:46

passed #769070
docker windows 1607
build vs2017 x86

00:10:49

 
  Test
passed #770572
check fedora

00:03:28

passed #769079
integration testsuites fedora

00:38:58

passed #769083
valgrind bad

00:12:02

passed #769080
valgrind base

00:08:36

passed #769084
valgrind ges

00:07:10

passed #769081
valgrind good

00:18:54

passed #769082
valgrind ugly

00:02:30

failed #769078
check fedora

00:03:36

 
  Integrate
passed #769085
cross-android universal examples

00:07:40

manual #769087
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #769086
documentation

00:05:47