Fix opcache build

[arnaud-lb]

In the absence of PHP_ARG_WITH([opcache],, the value of ext_shared is not initialized while processing directives of ext/opcache/config.m4, causing PHP_EVAL_LIBLINE() to add libs to OPCACHE_SHARED_LIBADD instead of LIBS.

For example, running ./configure --with-dbmaker=shared --with-capstone will set ext_shared=yes before processing opcache. In this case, -lcapstone is added to OPCACHE_SHARED_LIBADD.

Fixing by setting libs-variable to LIBS.

@remicollet could you confirm this fixes the issue?

[arnaud-lb]

Closing in favor of #19301

[remicollet]

duplicate of #19301 ;)

And yes I confirm the fix
Tested (RPM build) on Fedora 41, 42, RHEL 8, 9, 10 on x86_64 and aarch64