Only build in Debug mode when the debug runtime libraries are used. This fixes the following warning:
LINK : warning LNK4098: defaultlib 'MSVCRTD' conflicts with use of other libs; use /NODEFAULTLIB:library

This allows us to use generator expressions for the
library name to also fix the build on Windows.

- Avoid installation of empty lib/pkgconfig directory
- Use PCRE_PCRE_LIBRARY and PCRE_PCREPOSIX_LIBRARY variables

Fixes: ROOT-9864