This commit enables the building of the cgroup code. This is only built
when the cgroupv2 filesystem is found.

If cgroupv2 support is found then

  cgroupv2: .................. YES

will be printed by ./configure

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

It's for the introduction of njs support.
For each option that supports native variable and JS template literals introduced next,
it's unified as template string.

No functional changes.

This fixes an issue addressed in 651f5a37f5b8 on FreeBSD 12.

The problem manifested itself as:

configuring Ruby module
checking for -fdeclspec ... found
checking for Ruby library ... not found
checking for Ruby library in /usr/local/lib ... not found

./configure: error: no Ruby found.

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>

'install -d' has an issue compared to 'mkdir -p':  it doesn't
respect existing directories.  It will set the ownership, file
mode, and SELinux contexts (and any other property that would be
set by install(1) to a newly-created directory), overwriting any
existing properties of the existing directory.

'mkdir -p' doesn't have this issue:  it is a no-op if the
directory exists.  However, it's not an ideal solution either,
since it can't be used to set the properties (owner, mode, ...) of
a newly-created directory.

Therefore, the best solution is to use install(1), but only after
making sure that the directory doesn't exist with test(1).

Reported-by: Andrew Clayton <a.clayton@nginx.com>
Reported-by: Alejandro Colomar <alx@nginx.com>
Closes: <https://github.com/nginx/unit/issues/769>
Signed-off-by: Alejandro Colomar <alx@nginx.com>
Tested-by: Andrew Clayton <a.clayton@nginx.com>
Reviewed-by: Andrew Clayton <a.clayton@nginx.com>