Fix .gitignore to catch a few files created by Windows build process.

Also a few other minor changes to the file.

Author: c-lipka

.gitignore

@@ -150,6 +150,11 @@ $RECYCLE.BIN/
 # POV-Ray Windows build artefacts
 # ===============================
 
+/libraries/ilmbase/Half/eLut.h
+/libraries/ilmbase/Half/toFloat.h
+/libraries/png/pnglibconf.h
+/libraries/tiff/libtiff/tif_config.h
+/libraries/tiff/libtiff/tiffconf.h
 /windows/vs*/.vs/
 /windows/vs*/bin32/
 /windows/vs*/bin64/
@@ -215,12 +220,11 @@ $RECYCLE.BIN/
 # Directories created by configure script
 .deps/
 
-# Files created by make in main directory
-/povray.ini
-# Files created by make in specific subdirectories
+# Files created by make
 /unix/povray
-# Files created by make in various subdirectories
 .dirstamp
+# Files created by make check
+/povray.ini
 
 # ===========================
 # POV-Ray meta-build detritus
@@ -251,3 +255,10 @@ $RECYCLE.BIN/
 /tools/windows/htmlhelp/output
 /tools/windows/htmlhelp/debug.txt
 /tools/windows/htmlhelp/filelist.txt
+
+# ========================
+# GitHub Actions artefacts
+# ========================
+
+# Used specifically by our own workflows
+/artifact*