lirc: perform PYTHONPATH substitution with a patch

It is complex enough for a patch, and it is too complex for a
`substituteInPlace` command. A sed command can become a no-op and is
much harder to maintain.

pkgs/by-name/li/lirc/package.nix

@@ -51,15 +51,14 @@ stdenv.mkDerivation (finalAttrs: {
     # --with-systemdsystemunitdir
     # https://sourceforge.net/p/lirc/tickets/385/
     ./ubuntu.diff
+
+    # fix overriding PYTHONPATH
+    ./pythonpath.patch
   ];
 
   postPatch = ''
     patchShebangs .
 
-    # fix overriding PYTHONPATH
-    sed -i 's,^PYTHONPATH *= *,PYTHONPATH := $(PYTHONPATH):,' \
-      Makefile.in
-
     # Pull fix for new pyyaml pending upstream inclusion
     #   https://sourceforge.net/p/lirc/git/merge-requests/39/
     substituteInPlace python-pkg/lirc/database.py \

pkgs/by-name/li/lirc/pythonpath.patch

@@ -0,0 +1,13 @@
+diff --git i/Makefile.in w/Makefile.in
+index d039ed3e9d6a..c40e6eac56ae 100644
+--- i/Makefile.in
++++ w/Makefile.in
+@@ -493,7 +493,7 @@ AUTOMAKE_OPTIONS = 1.5 check-news dist-bzip2 -Wno-portability \
+ 
+ PYTHONPATH1 = $(abs_top_srcdir)/python-pkg/lirc:
+ PYTHONPATH2 = $(abs_top_srcdir)/python-pkg/lirc/lib/.libs
+-PYTHONPATH = $(PYTHONPATH1):$(PYTHONPATH2)
++PYTHONPATH := $(PYTHONPATH):$(PYTHONPATH1):$(PYTHONPATH2)
+ PYLINT = python3-pylint
+ pylint_template = {path}:{line}: [{msg_id}({symbol}), {obj}] {msg}
+ GIT_COMMIT = $(shell git log -1 --pretty=format:%h || echo UNKNOWN)