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)
ub right'></td></tr></table>
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<div class='path'>path: <a href='/guix/log/?id=6264a7cca0982ab4694bc71d2944c383dd571ac6'>root</a>/<a href='/guix/log/guix.scm?id=6264a7cca0982ab4694bc71d2944c383dd571ac6'>guix.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/guix.scm?id=6264a7cca0982ab4694bc71d2944c383dd571ac6&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-07-12 01:17:45 +0200'>2022-07-12</span></td><td><a href='/guix/commit/guix.scm?id=4574ef267e543bcac97d9c400f30cb731df29181'>guix: Really export 'define-public'.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is a followup to 3858275962f1ac567b5dce2355035f9eddea0a05, which
didn't have any effect.

* guix.scm: Explicitly use (guix packages) so #:re-export-and-replace
is effective.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-09-14 11:50:58 +0200'>2021-09-14</span></td><td><a href='/guix/commit/guix.scm?id=3858275962f1ac567b5dce2355035f9eddea0a05'>guix: Replace 'define-public'.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is a followup to 8531997d2a1e10d574a6e9ab70bc86ade6af4733.

* guix.scm: Use #:re-export-and-replace for 'define-public'.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-06-29 00:00:13 +0200'>2021-06-29</span></td><td><a href='/guix/commit/guix.scm?id=e059adcb6001ab0353d969b24030d0c342f8830e'>(guix) no longer pulls in (guix ftp-client).</a><span class='msg-avail'>...<span class='msg-tooltip'>That choice was made in 2014 and never made much sense.

* guix.scm (%public-modules): Remove 'ftp-client'.
</span></span></td><td>Ludovic Courtès</td></tr>