AC_CHECK_HEADERS_ONCE: honor current AC_LANG

Previously, AC_CHECK_HEADERS_ONCE collected a list of header names
to check, but ran the checks using the AC_LANG that was active
during the first encounter of the macro.  In practice, this is
usually the C language, and we haven't had actual reports of projects
attempting to use AC_CHECK_HEADERS_ONCE across multiple languages,
rather this was discovered by code inspection.

With this patch, the code now tracks a separate per-language list of
names to check.  Note, however, that it is only possible to check for
a given header name in one language; attempting to add a name again
under AC_CHECK_HEADERS_ONCE while a different language is active is a
no-op (this still makes sense because the side-effect of defining
the CPP macro HAVE_HEADER does not include a language prefix).

* lib/autoconf/headers.m4 (_AC_CHECK_HEADER_ONCE)
(_AC_HEADERS_EXPANSION):
* NEWS: Mention it.

Signed-off-by: Eric Blake <eblake@redhat.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/NEWS b/NEWS
index 532e15e7..b82c1e3c 100644
--- a/NEWS
+++ b/NEWS
@@ -111,9 +111,9 @@ GNU Autoconf NEWS - User visible changes.
   useful effect is to trigger those checks, with this macro.  It is
   unlikely to be useful otherwise.
 
-- The AC_CHECK_FUNCS_ONCE macro now supports use with multiple
-  languages, rather than forcing all checks in the language used by
-  the first encounter of the macro.
+- The AC_CHECK_FUNCS_ONCE and AC_CHECK_HEADERS_ONCE macros now support
+  use with multiple languages, rather than forcing all checks in the
+  language used by the first encounter of the macro.
 
 ** Man pages for config.guess and config.sub are no longer provided.
 They were moved to the master source tree for config.guess and config.sub.