[gimp] meson: allow non-dist non-git builds but mark them unsupported.

[Jehan <jehanp src gnome org>]

commit 8b5060349a95ca552cc1afa9f9c6f4becc2fe7f6
Author: Jehan <jehan girinstud io>
Date:   Thu Nov 21 12:21:08 2019 +0100

    meson: allow non-dist non-git builds but mark them unsupported.
    
    Reported by Jan Tojnar as a comment to commit e4c7fc23 that builds from
    simple archives of the repo contents (without the .git tree) are
    currently broken. Well this is normal, as we only support builds from
    release versions or from the repository where we can extract a git hash.
    Any other kind of nightly build can be from any commit out of thousands
    and is maintenance hell.
    
    To be nice though, let's unbreak such builds, but they will be clearly
    marked as unsupported (warning at configure time + the extract commit
    hash will now be labelled "unknown (unsupported)", which will be a
    visible string in About and on unstable version canvases).
    Basically do so at your own risk.
    
    Also generate INSTALL all the time (not sure why it was only generated
    in non-git case).

 meson.build | 45 ++++++++++++++++++++++++++++++---------------
 1 file changed, 30 insertions(+), 15 deletions(-)
---
diff --git a/meson.build b/meson.build
index a1ec1f67b6..2a0191bcba 100644
--- a/meson.build
+++ b/meson.build
@@ -16,9 +16,9 @@ conf = configuration_data()
 warnings = []
 
 # git-version.h is already present and not generated if dist tarball
-is_dist_tarball = run_command('python', '-c',
+is_git_repository = run_command('python', '-c',
   'import sys,os; sys.exit(0 if os.path.exists(".git") else 1)'
-).returncode() == 1
+).returncode() == 0
 
 ################################################################################
 # Project info
@@ -1431,30 +1431,47 @@ endif
   #####   ####  #    # #      #  ####     #       # ###### ######  ####
 
 
-if is_dist_tarball
-  gitversion_h = files('git-version.h')
-else
+has_version_h = run_command('python', '-c',
+  'import sys,os; sys.exit(0 if os.path.exists("git-version.h") else 1)'
+).returncode() == 0
+if is_git_repository or not has_version_h
   gitversion_h1 = vcs_tag(
     input : 'app/git-version.h.in',
     output: 'git-version.h.in.1',
     command: [ 'git', 'describe', '--always', ],
     replace_string: '@GIMP_GIT_VERSION@',
-    fallback: '',
+    fallback: 'unknown (unsupported)',
   )
   gitversion_h2 = vcs_tag(
     input : gitversion_h1,
     output: 'git-version.h.in.2',
     command: [ 'git', 'rev-parse', '--short', 'HEAD', ],
     replace_string: '@GIMP_GIT_VERSION_ABBREV@',
-    fallback: '',
+    fallback: 'unknown (unsupported)',
   )
   gitversion_h = vcs_tag(
     input : gitversion_h2,
     output: 'git-version.h',
     command: [ 'git', 'log', '-n1', '--date=format:%Y', '--pretty=%cd', ],
     replace_string: '@GIMP_GIT_LAST_COMMIT_YEAR@',
-    fallback: '',
+    fallback: 'unknown (unsupported)',
   )
+  if not is_git_repository
+  # We create git-version.h but know it will be useless because we are
+  # not in a git repository. Output a warning.
+  git_warning = '''
+
+        UNSUPPORTED BUILD!
+
+        This is not a distribution tarball (git-version.h missing) nor is it
+        a git repository. Therefore we have no reference for debugging.
+        Please either use release tarballs or build from the repository.
+  '''
+  warning(git_warning)
+  warnings += git_warning
+  endif
+else
+  gitversion_h = files('git-version.h')
 endif
 
 install_conf = configuration_data()
@@ -1492,13 +1509,11 @@ install_conf.set('WEBP_REQUIRED_VERSION',         webp_minver)
 install_conf.set('WMF_REQUIRED_VERSION',          wmf_minver)
 install_conf.set('XGETTEXT_REQUIRED_VERSION',     '0.19')
 
-if not is_dist_tarball
-  INSTALL = configure_file(
-    input : 'INSTALL.in',
-    output: 'INSTALL',
-    configuration: install_conf
-  )
-endif
+INSTALL = configure_file(
+  input : 'INSTALL.in',
+  output: 'INSTALL',
+  configuration: install_conf
+)
 
 
 configure_file(