Add an environment variable to python-sip that extends its search
directories for .sip files.

It seems that we cannot easily change the destination folder of these
files though, so this variable must be set on a per-package basis (and
non through search-path).

--- sip/sipbuild/builder.py	2023-03-22 09:06:37.588792878 +0100
+++ sip/sipbuild/builder.py	2023-03-22 09:10:35.830181134 +0100
@@ -254,6 +254,10 @@
                     os.path.join(project.target_dir,
                             project.get_bindings_dir()))
 
+            # Add extra bindings from environment for GNU Guix.
+            if 'SIP_INCLUDE_DIRS' in os.environ:
+                sip_include_dirs.extend(os.environ['SIP_INCLUDE_DIRS'].split(os.pathsep))
+
             # Generate the sip.h file for the shared sip module.
             copy_sip_h(abi_major_version, project.build_dir,
                     project.sip_module, version_info=project.version_info)
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<tr><td><span title='2019-05-03 17:47:30 +0200'>2019-05-03</span></td><td><a href='/guix/commit/po/packages/da.po?id=adb5506b2cfb47f696d66a03ede897e9bfa829bb'>nls: Update da translations.</a></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-02-13 21:40:02 +0100'>2019-02-13</span></td><td><a href='/guix/commit/po/packages/da.po?id=d093f3e4a4dd155969499ac2dc5fdb069a611c75'>nls: Update 'da' translation.</a></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2018-06-18 23:19:51 +0200'>2018-06-18</span></td><td><a href='/guix/commit/po/packages/da.po?id=f202292217c0a5588d1c281014e9453ff0343c19'>nls: Update 'da' translation.</a></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-05-23 13:36:58 -0400'>2017-05-23</span></td><td><a href='/guix/commit/po/packages/da.po?id=db375739dac31360f5ca945ff06edfd952595cc5'>nls: Update 'da' translation.</a></td><td>Leo Famulari</td></tr>