Since commit
52ca3d3fe61 ("toplevel: unify the GCC and GDB/binutils .editorconfig
files"), .editorconfig has settings for .ac and .m4 files:
...
[*.{ac,m4}]
indent_style = tab
indent_size = 2
trim_trailing_whitespace = true
...
There are no setting for those files in .gitattributes, so the whitespace
attribute defaults to trailing-space (shorthand for blank-at-eol,
blank-at-eof) and space-before-tab.
Since according to .editorconfig the indentation style is tab, add
indent-with-non-tab as well.
Since aclocal.m4 is generated, unset the whitespace attribute. Likewise for
configure.
*.[cChH] whitespace=space-before-tab,indent-with-non-tab,trailing-space
*.exp whitespace=space-before-tab,indent-with-non-tab,trailing-space
*.tcl whitespace=space-before-tab,indent-with-non-tab,trailing-space
+
+# Autoconf.
+
+*.ac whitespace=space-before-tab,indent-with-non-tab,trailing-space
+*.m4 whitespace=space-before-tab,indent-with-non-tab,trailing-space
+
+# Generated files.
+
+configure -whitespace
+aclocal.m4 -whitespace