Future releases of GCC are planning to remove[0] default support for
some old features that were removed from C99 but GCC still accepts.

We can test for these changes by using the following -Werror=
directives

  -Werror=implicit-int
  -Werror=implicit-function-declaration
  -Werror=int-conversion
  -Werror=strict-prototypes
  -Werror=old-style-definition

Doing so revealed an issue with the auto/ tests in that the test
programs always define main as

  int main()

rather than

  int main(void)

which results in a bunch of errors like

build/autotest.c:3:23: error: function declaration isn't a prototype [-Werror=strict-prototypes]
    3 |                   int main() {
      |                       ^~~~
build/autotest.c: In function 'main':
build/autotest.c:3:23: error: old-style function definition [-Werror=old-style-definition]

The fix was easy, it only required fixing the main prototype with

  find -type f -exec sed -i 's/int main() {/int main(void) {/g' {} \;

Regardless of these upcoming GCC changes, this is probably a good thing
to do anyway for correctness.

[0]: https://fedoraproject.org/wiki/Changes/PortingToModernC

Link: <https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/message/CJXKTLXJUPZ4F2C2VQOTNMEA5JAUPMBD/>
Link: <https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/message/6SGHPHPAXKCVJ6PUZ57WVDQ5TDBVIRMF/>
Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Thanks to @wujjpp.

This closes #490 PR on GitHub.

This variant will be more interoperable across various systems
and it's already used in Ruby module.

Otherwise, configure tests fail on NetBSD with:

  gcc: Missing argument for -Wl,-rpath

Now it's possible to disable default bind mounts of
languages by setting:

  {
      "isolation": {
           "automount": {
               "language_deps": false
           }
     }
  }

In this case, the user is responsible to provide a "rootfs"
containing the language libraries and required files for
the application.