[libgit2-glib] meson_vapi_link.py: Improve calculation of vapidir

[Alberto Fanjul <albfan src gnome org>]

commit 170e1c31dad2208e3ce659628efdbf0623ebbda7
Author: Jan Alexander Steffens (heftig) <jan steffens gmail com>
Date:   Sun Mar 1 17:33:12 2020 +0000

    meson_vapi_link.py: Improve calculation of vapidir
    
    The script gets the *absolute* datadir passed as its argument. This
    means os.path.join will throw away the MESON_INSTALL_DESTDIR_PREFIX,
    breaking the symlinking when DESTDIR is set.
    
    Also handle the case where the datadir is not inside the prefix by
    prepending the destdir ourselves. We make the datadir relative to the
    root by stripping the drive letter (if any) and the initial separator.

 meson.build        | 2 +-
 meson_vapi_link.py | 9 ++++++++-
 2 files changed, 9 insertions(+), 2 deletions(-)
---
diff --git a/meson.build b/meson.build
index 3a423b3..6d314b5 100644
--- a/meson.build
+++ b/meson.build
@@ -135,7 +135,7 @@ if enable_vapi
   assert(enable_gir, 'vapi support was requested, but introspection support is mandatory.')
   assert(add_languages('vala', required: false), 'vapi support was requested, but vala not found.')
 
-  meson.add_install_script('meson_vapi_link.py', get_option('datadir'))
+  meson.add_install_script('meson_vapi_link.py', libgit2_glib_datadir)
 endif
 
 # Check for libgit2 ssh support
diff --git a/meson_vapi_link.py b/meson_vapi_link.py
index 58e0f6b..d7fc729 100644
--- a/meson_vapi_link.py
+++ b/meson_vapi_link.py
@@ -4,7 +4,14 @@ import os
 import subprocess
 import sys
 
-vapidir = os.path.join(os.environ['MESON_INSTALL_DESTDIR_PREFIX'], sys.argv[1], 'vala', 'vapi')
+datadir = sys.argv[1]
+
+destdir = os.environ.get('DESTDIR')
+if destdir:
+    datadir = os.path.splitdrive(datadir)[1].lstrip(os.path.sep)
+    datadir = os.path.join(destdir, datadir)
+
+vapidir = os.path.join(datadir, 'vala', 'vapi')
 os.chdir(vapidir)
 
 # FIXME: meson will not track the creation of these files