make install, on 64 bit, installs cmake files into lib instead of lib64 folder

[a4z]

Description

make install put all doctest*.cmake files into /install/prefix/lib/cmake/doctest,
but on some Linux platforms this should be /install/prefix/lib64/cmake/doctest

Steps to reproduce

on a Linux 64 bit distribution, where libraries files go into /usr/lib64

mkdir build
cd build
cmake -DCMAKE_CXX_FLAGS=-std=c++11 ../
make install DESTDIR=$(pwd)/zinst

The installed files will be

-- Installing: /where/ever/doctest/build/zinst/usr/local/include/doctest/doctest.h
-- Installing: /where/ever/doctest/build/zinst/usr/local/lib/cmake/doctest/doctestConfig.cmake
-- Installing: /where/ever/doctest/build/zinst/usr/local/lib/cmake/doctest/doctestConfigVersion.cmake
-- Installing: /where/ever/doctest/build/zinst/usr/local/lib/cmake/doctest/doctestTargets.cmake

Extra information

• doctest version: v2.3.1
• Operating System: RHEL 7.6
• Compiler+version: gcc 4.8, cmake 3.13

[onqtam]

merged the PR