fix: for exporters to work for build config, there may be two include dirs

For CMake / pkg-config configuration files to be used for an uninstalled
build, the include directory in the build directory isn't enough, if that
one is separate from the source directory.  The include directory in the
source directory must be accounted for too.

This includes some lighter refactoring of util/mkinstallvars.pl, with the
result that almost all variables in builddata.pm and installdata.pm have
become arrays, even though unnecessarily for most of them; it was simpler
that way.  The CMake / pkg-config templates are adapted accordingly.

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24918)