[meson] fix version header custom target for meson >= 0.57

Apparently custom_target no longer supports the @CURRENT_SOURCE_DIR@
and @SOURCE_DIR@ substitutions.
We now locate the script using find_program().
Since meson.project_source_dir() is not available in older meson version
there is a version check.

emper/include/meson.build

 emper_include = include_directories('.')
 
+versionHeaderScript = find_program('generateVersionHeader.sh')
+
+meson_version = meson.version().split('.')
+major = meson_version[0].to_int()
+minor = meson_version[1].to_int()
+if major > 0 or minor > 56
+	emper_version_h = custom_target(
+		'emper-version.h',
+		output: 'emper-version.h',
+		command: [
+			versionHeaderScript,
+			'generate',
+			meson.project_version(),
+			'@OUTPUT@',
+			meson.project_source_root(),
+		]
+	)
+else
 	emper_version_h = custom_target(
 		'emper-version.h',
 		output: 'emper-version.h',
 		command: [
-	'@CURRENT_SOURCE_DIR@/generateVersionHeader.sh',
+			versionHeaderScript,
 			'generate',
 			meson.project_version(),
 			'@OUTPUT@',
 			'@SOURCE_DIR@',
 		]
 	)
+endif
 emper_generated_files += emper_version_h