[gcab] build-sys: don't require git to build

[Marc-André Lureau <malureau src gnome org>]

commit b9972fb3329c2b170132a51694b06d0d8189efd5
Author: Marc-André Lureau <marcandre lureau redhat com>
Date:   Tue Feb 13 11:48:02 2018 +0100

    build-sys: don't require git to build
    
    Check git is available before running it.
    
    Fixes:
    https://bugzilla.gnome.org/show_bug.cgi?id=793406
    
    Signed-off-by: Marc-André Lureau <marcandre lureau redhat com>

 meson.build |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)
---
diff --git a/meson.build b/meson.build
index f2ae0d2..861df60 100644
--- a/meson.build
+++ b/meson.build
@@ -5,7 +5,11 @@ project('gcab', 'c',
   default_options : ['warning_level=2', 'c_std=c99'],
 )
 
-git_version = run_command(['git', 'describe', '--abbrev=4', 'HEAD']).stdout().strip().split('-')
+git_version = []
+git = find_program('git', required: false)
+if git.found()
+  git_version = run_command(git, 'describe', '--abbrev=4', 'HEAD').stdout().strip().split('-')
+endif
 
 # libtool versioning
 lt_current = '0'