CMake option to compile against an external PCRE library

Under terms of MDEV-5304 we upgraded PCRE to 8.34
but did not add a CMake options to compile against an
external PCRE library.
We should eventually add a CMake option.

The problem is that using an external PCRE is dangerous
because of a possible stack overrun, which we fix in the bundled
PCRE with the patch pcre/maria-patches/pcre_stack_guard.diff.

There are some options:
a. wait for the PCRE team to add some built-in means to watch the stack
b. use the current PCRE version and check the pattern against
nested parenthesis every time we call pcre_compile().