Improve git sha1 detection

meson.build

@@ -9,6 +9,7 @@ if get_option('buildtype').startswith('debug')
   is_debug_build = 1
 endif
 
+git_prog = find_program('git', required: false)
 cpp = meson.get_compiler('cpp')
 
 conf = configuration_data()
@@ -26,12 +27,22 @@ project_config_file = configure_file(
 	output: 'config.h',
 	configuration: conf
 )
-gitrev_config_file = vcs_tag(
-	command: ['git', 'rev-parse', 'HEAD'],
-	input: 'src/git_version.h.in',
-	output: 'git_version.h',
-	fallback: 'n/a'
-)
+if git_prog.found()
+	gitrev_config_file = vcs_tag(
+		command: [git_prog.path(), 'rev-parse', 'HEAD'],
+		input: 'src/git_version.h.in',
+		output: 'git_version.h',
+		fallback: ''
+	)
+else
+	conf = configuration_data()
+	conf.set('VCS_TAG', '')
+	gitrev_config_file = configure_file(
+		input: 'src/git_version.h.in',
+		output: 'git_version.h',
+		configuration: conf
+	)
+endif
 app_config_model = files('src/app_config.h.in')
 
 cxxopts_incl = include_directories('subprojects/cxxopts/include', '.')