[meson] update generated headers only when the result is different

This way it won't ruin incremental builds.

We need a better way to declare source altering tasks
https://github.com/mesonbuild/meson/issues/7156
yet this is good enough despite being not idiomatic.

It is however not that smooth yet as the change may is detected on the
next meson run. One of course can issue ./gen-ragel-artifacts.py
manually for better experience before running meson.

src/gen-hb-version.py

diff --git a/src/gen-hb-version.py b/src/gen-hb-version.py
index 70e69d8..e296424 100755
--- a/src/gen-hb-version.py
+++ b/src/gen-hb-version.py
@@ -10,10 +10,18 @@
 input = sys.argv[2]
 output = sys.argv[3]
 
-with open (output, "wb") as output_file, open (input, "r", encoding='utf-8') as input_file:
-	output_file.write (input_file.read ()
+with open (input, "r", encoding='utf-8') as input_file:
+	generated = (input_file.read ()
 		.replace ("@HB_VERSION_MAJOR@", major)
 		.replace ("@HB_VERSION_MINOR@", minor)
 		.replace ("@HB_VERSION_MICRO@", micro)
 		.replace ("@HB_VERSION@", version)
 		.encode ())
+
+	with open (output, "rb") as current_file:
+		current = current_file.read()
+
+	# write only if is changed
+	if generated != current:
+		with open (output, "wb") as output_file:
+			output_file.write ()

src/gen-ragel-artifacts.py

diff --git a/src/gen-ragel-artifacts.py b/src/gen-ragel-artifacts.py
index 2f176bb..53a3038 100755
--- a/src/gen-ragel-artifacts.py
+++ b/src/gen-ragel-artifacts.py
@@ -1,11 +1,12 @@
 #!/usr/bin/env python3
 
-import io, os, re, sys, subprocess, shutil
+import io, os, re, sys, subprocess, shutil, tempfile
 
-if len (sys.argv) < 3:
+os.chdir(os.path.dirname(__file__))
+
+if len (sys.argv) < 2:
 	ragel_sources = [x for x in os.listdir ('.') if x.endswith ('.rl')]
 else:
-	os.chdir(sys.argv[1])
 	ragel_sources = sys.argv[2:]
 
 ragel = shutil.which ('ragel')
@@ -17,6 +18,23 @@
 if not len (ragel_sources):
 	exit (77)
 
+tempdir = tempfile.mkdtemp ()
+
 for rl in ragel_sources:
 	hh = rl.replace ('.rl', '.hh')
-	subprocess.Popen ([ragel, '-e', '-F1', '-o', hh, rl]).wait ()
+	shutil.copy (rl, tempdir)
+	# writing to stdout has some complication on Windows
+	subprocess.Popen ([ragel, '-e', '-F1', '-o', hh, rl], cwd=tempdir).wait ()
+
+	generated_path = os.path.join (tempdir, hh)
+	with open (generated_path, "rb") as temp_file:
+		generated = temp_file.read()
+
+	with open (hh, "rb") as current_file:
+		current = current_file.read()
+
+	# overwrite only if is changed
+	if generated != current:
+		shutil.copyfile (generated_path, hh)
+
+shutil.rmtree(tempdir)

src/meson.build

diff --git a/src/meson.build b/src/meson.build
index c17513b..698f006 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -289,16 +289,13 @@
 
 ragel = find_program('ragel', required: false)
 if not ragel.found()
-  message('You have to install ragel if you are going to develop HarfBuzz itself')
+  warning('You have to install ragel if you are going to develop HarfBuzz itself')
 else
-  foreach rl : hb_base_ragel_sources
-    hh = rl.split('.rl')[0] + '.hh'
-    custom_target(hh,
-      build_by_default: true,
-      depfile: rl,
-      output: hh,
-      command: [find_program('gen-ragel-artifacts.py'), meson.current_source_dir(), rl])
-  endforeach
+  custom_target('ragel header files',
+    build_by_default: true,
+    input: hb_base_ragel_sources,
+    output: '.ragel-artifacts',
+    command: [find_program('gen-ragel-artifacts.py')] + hb_base_ragel_sources)
 endif
 
 custom_target('harfbuzz.cc',