CMake: Installation directory for include files in apr 1.7.x

Hi,

I am currently working on CMake and vcpkg [1].

One problem that I noticed is that APR 1.7.x (and APR-Util 1.6.x)
install include files to INSTALL_PREFIX/include. While on other
platforms include files installed to INSTALL_PREFIX/include/apr-1 (and
apr-2 for apr-trunk).

This problem is fixed in apr-trunk: CMake based builds install include
files to INSTALL_PREFIX/include/apr-2.

Installation to root include is not recommended by vcpkg. This can
also introduce problems when apr-1 and apr-2 are installed
side-by-side.

I want to fix this in APR 1.7.x, but this could break users who rely
that APR is installed to INSTALL_PREFIX/include.

I see several options:
1. Just change to INSTALL_PREFIX/include/apr-1
2. Add a compile-time CMake option like APR_INSTALL_INCLUDE_TO_ROOT
and enable it by default for apr 1.7.x, but disable for vcpkg builds.

For me it seems that (2) is a safer way, but maybe it's overkill?

[1] https://vcpkg.io/en/

-- 
Ivan Zhakov