If check_c_source_compiles() succeeded (HAVE_WORKING_LIBINTL is set)
then the result of find_xxx() doesn't matter. This happens on systems
(linux+glibc) where libintl is available passively.

This allows `find_package(LibIntl REQUIRED)` to work and will still
correctly fail (REQUIRED) on systems lacking libintl.

It only shortens the traceback a bit for when a module is not found
though, only removing the "(command line):1: in main chunk" (with
lua5.2).

This helps to figure out what the problem is, e.g. in my case I have
lua51-mpack installed to be used with luajit, but it is broken (missing
libmpack).

With this patch you get:

    -- Checking Lua interpreter /usr/bin/luajit
    /usr/bin/luajit: error loading module 'mpack' from file '/usr/lib/lua/5.1/mpack.so':
            libmpack.so.0: cannot open shared object file: No such file or directory
    stack traceback:
            [C]: at 0x55fcf0166fb0
            [C]: in function 'require'
            (command line):1: in main chunk
            [C]: at 0x55fcf01188a0
    -- [/usr/bin/luajit] The 'mpack' lua package is required for building Neovim
    -- Checking Lua interpreter /usr/bin/lua5.1
    /usr/bin/lua5.1: error loading module 'mpack' from file '/usr/lib/lua/5.1/mpack.so':
            libmpack.so.0: cannot open shared object file: No such file or directory
    stack traceback:
            [C]: ?
            [C]: in function 'require'
            (command line):1: in main chunk
            [C]: ?
    -- [/usr/bin/lua5.1] The 'mpack' lua package is required for building Neovim
    -- Checking Lua interpreter /usr/bin/lua5.2
    /usr/bin/lua5.2: (command line):1: module 'mpack' not found:
            no field package.preload['mpack']
            no file '/usr/share/lua/5.2/mpack.lua'
            no file '/usr/share/lua/5.2/mpack/init.lua'
            no file '/usr/lib/lua/5.2/mpack.lua'
            no file '/usr/lib/lua/5.2/mpack/init.lua'
            no file './mpack.lua'
            no file '/usr/lib/lua/5.2/mpack.so'
            no file '/usr/lib/lua/5.2/loadall.so'
            no file './mpack.so'
    stack traceback:
            [C]: in function 'require'
            (command line):1: in main chunk
            [C]: in ?
    -- [/usr/bin/lua5.2] The 'mpack' lua package is required for building Neovim
    -- Checking Lua interpreter /usr/bin/lua
    /usr/bin/lua: (command line):1: module 'mpack' not found:
            no field package.preload['mpack']
            no file '/usr/share/lua/5.3/mpack.lua'
            no file '/usr/share/lua/5.3/mpack/init.lua'
            no file '/usr/lib/lua/5.3/mpack.lua'
            no file '/usr/lib/lua/5.3/mpack/init.lua'
            no file './mpack.lua'
            no file './mpack/init.lua'
            no file '/usr/lib/lua/5.3/mpack.so'
            no file '/usr/lib/lua/5.3/loadall.so'
            no file './mpack.so'
    stack traceback:
            [C]: in function 'require'
            (command line):1: in main chunk
            [C]: in ?
    -- [/usr/bin/lua] The 'mpack' lua package is required for building Neovim
    CMake Error at CMakeLists.txt:459 (message):
      A suitable Lua interpreter was not found.

While this makes it more verbose for the expected error case ("module
'mpack' not found"), the behavior before this patch hides too much.
This is the old output:

    -- Checking Lua interpreter /usr/bin/luajit
    -- [/usr/bin/luajit] The 'mpack' lua package is required for building Neovim
    -- Checking Lua interpreter /usr/bin/lua5.1
    -- [/usr/bin/lua5.1] The 'mpack' lua package is required for building Neovim
    -- Checking Lua interpreter /usr/bin/lua5.2
    -- [/usr/bin/lua5.2] The 'mpack' lua package is required for building Neovim
    -- Checking Lua interpreter /usr/bin/lua
    -- [/usr/bin/lua] The 'mpack' lua package is required for building Neovim
    CMake Error at CMakeLists.txt:459 (message):
      A suitable Lua interpreter was not found.

This is for when the whole configuration runs (i.e. after `make
distclean`), afterwards only one Lua interpreter gets checked only.

Reduces noise in :version message.

in case LUA_PATH does not contain `./?.lua`, busted can not load lua test scripts.
(for instance on nixos). Thus we make it explicit.

closes #7239

The old behaviour was to set CMAKE_INSTALL_MANDIR to /usr/local/man
when MANPREFIX wasn't defined. This caused mismatching installation
paths when the installation prefix wasn't /usr/local.

This fix explicitely checks that the prefix is /usr/local to change
the value of CMAKE_INSTALL_MANDIR, and uses the default behaviour
otherwise, as /usr/local is the exception rather than the norm
(as per man hier(7)).