Ticket History #8928: Fix three configure tests for Xcode 12

On recent versions of clang, calling functions without first having
imported their headers is an error. This causes the
constructor/destructor configure check to fail; it doesn't include any
headers, and a function it uses goes undefined. Sample error:

configure:5947: checking for constructor/destructor attribute support
configure:5975: clang -o conftest -g -O2 -fno-common
-Wl,-search_paths_first conftest.c -lresolv >&5
conftest.c:25:15: error: implicit declaration of function 'unlink' is
invalid in C99 [-Werror,-Wimplicit-function-declaration]
void foo1() { unlink("conftest.1"); }
^

Patch:

diff --git a/src/aclocal.m4 b/src/aclocal.m4
index 2394f7e..811e987 100644
--- a/src/aclocal.m4
+++ b/src/aclocal.m4
@@ -1606,7 +1606,8 @@ fi
a=no
b=no
# blindly assume we have 'unlink'...
-AC_TRY_RUN([void foo1() __attribute__((constructor));
+AC_TRY_RUN([#include <unistd.h>
+void foo1() __attribute__((constructor));
void foo1() { unlink("conftest.1"); }
void foo2() __attribute__((destructor));
void foo2() { unlink("conftest.2"); }

Additional notes:

* The compiler line doesn't include -Werror in the options, but the error message does, which is weird.
* Using godbolt.org, recent versions of the test program don't seem to give an error on the test program by default.
* The presence of -fno-common in the compiler options suggests macOS (aclocal.m4 line 589).
* If I explicitly configure with CFLAGS=-Werror, lots of tests go wrong.  TRY_WARN_CC_FLAG_1 errors out entirely (fixable by changing "1;" to ";" in the AC_TRY_COMPILE program arguments), Tcl isn't found, functions like vsprintf and strdup aren't found, etc..
* A search of the autoconf bug list suggests that making configure work with CFLAGS=-Werror is not a design goal.

It's conceivable that Apple has modified clang to error out on implicit-function-declaration by default, and that the diagnostic includes Werror due to the nature of the local modification.  If so, there may be two other tests giving incorrect results.  My (Linux) config.log reports an implicit declaration warning for Tcl_CreateInterp in the tcl tests and for printf in the in6addr_any test.  AC_CHECK_FUNC doesn't seem to generate implicit declaration warnings all the time because it sticks a "char fnname ();" in the body, but is does generate "conflicting types for built-in function" warnings when the function being tested for has a compiler builtin.
 

Thanks for the additional information.  That kind of compiler change is hostile to autoconf, whose developers (looking over conversations on their mailing list over many years) do not feel that they can avoid warnings across all compilers in its code snippets.  There does appear to be some effort in the autoconf core macros to avoid implicit function declaration warnings, but of course custom tests can fail, as in this case.

I have identified three tests that produce the wrong answer with -Werror=implicit-function-declarations: this one, the test for Tcl, and the test for in6addr_any.  I will work on a patch for all three.