Fallout from _AC_UNDECLARED_WARNING in autoconf 2.70beta

[Stephan Bergmann]

One issue I encountered in the wild (when building LibreOffice, with its 
zoo of bundled 3rd-party projects) with (the latest git trunk for) 
autoconf 2.70beta is related to how gpgme's configure.ac 
(<https://git.gnupg.org/cgi-bin/gitweb.cgi?p=gpgme.git;a=blob;f=configure.ac;h=03c260cd759dfda37ef047a143b6875ae81305cb;hb=HEAD>) 
is organized.  (It may or may not be considered a regression in 
autoconf, or a bug in gpgme, hence I'm only writing here for now, not 
filing an issue.  The relevant gpgme mailing list is on CC.)

That configure.ac checks whether one specific GCC 
-Wno-missing-field-initializers flag is supported (l. 606), based on 
that one check optimistically goes ahead and adds a whole bunch of 
-Wno-... flags to CFLAGS, and only then (l. 661) happens to call 
_AC_UNDECLARED_WARNING (via gnupg_REPLACE_TTYNAME_R -> 
AC_CHECK_DECLS_ONCE -> _AC_CHECK_DECL_BODY) for the first time.

If that configure is invoked with CC=clang supporting 
-Wno-missing-field-initializers but not supporting e.g. 
-Wno-format-truncation (which is among the bunch of flags optimistically 
added to CFLAGS), that was not much of an issue in the past; invocations 
of clang kept emitting noisy but harmless

> warning: unknown warning option '-Wno-format-truncation' [-Wunknown-warning-option]

lines.  But now, _AC_CHECK_DECL_BODY fails hard with

> configure:21628: checking how clang reports undeclared, standard C functions
> configure:21645: clang -c -g -O2 -Wall -Wcast-align -Wshadow -Wstrict-prototypes -Wno-format-y2k -Wno-missing-field-initializers -Wno-sign-compare -Wno-format-zero-length -Wno-format-truncation -Wno-sizeof-pointer-div  conftest.c >&5
> warning: unknown warning option '-Wno-format-truncation' [-Wunknown-warning-option]
> conftest.c:51:8: warning: implicitly declaring library function 'strchr' with type 'char *(const char *, int)' [-Wimplicit-function-declaration]
> (void) strchr;
>        ^
> conftest.c:51:8: note: include the header <string.h> or explicitly provide a declaration for 'strchr'
> 2 warnings generated.
> configure:21645: $? = 0
> configure:21675: clang -c -g -O2 -Wall -Wcast-align -Wshadow -Wstrict-prototypes -Wno-format-y2k -Wno-missing-field-initializers -Wno-sign-compare -Wno-format-zero-length -Wno-format-truncation -Wno-sizeof-pointer-div  conftest.c >&5
> warning: unknown warning option '-Wno-format-truncation' [-Wunknown-warning-option]
> 1 warning generated.
> configure:21675: $? = 0
> configure:21679: error: in `/home/sbergman/git.gnupg.org/gpgme':
> configure:21681: error: cannot detect from compiler exit status or warnings

One workaround is to move the gnupg_REPLACE_TTYNAME_R before the 
-Wno-missing-field-initializers etc. block in configure.ac.  And maybe 
there would be a dedicated macro that can be called early to set up 
_AC_UNDECLARED_WARNING prior to modifying CFLAGS?