bug#18926: cmake-build-system should set CMAKE_INSTALL_LIBDIR and CMAKE_PREFIX_PATH

[Andreas Enge <andreas@enge.fr>]

On Sun, Mar 01, 2015 at 03:35:51PM +0100, Ludovic Courtès wrote:
> That’s not currently possible using the search path mechanism (and I
> can’t imagine such weird semantics.)

I think the semantics is relatively clear: Match file names with a regular
expression and, if there is a match, add the result to a search path.
It is much like search-replace with regular expressions.

But of course, the search path could become very long: Essentially each
package built with cmake would contribute an entry.

> I can imagine two solutions, in order of preference:
>    1. Find CMAKE_ environment variables that can be set using the search
>       path mechanism.  Many environment variables are documented, is
>       there really none that we can use?

The problem here is that the .cmake files are in a subdirectory which is
named after the package. I think our current search path specification
cannot handle this, since we can only use it with fixed prefixes of a full
file location. One possibility would be to use CMAKE_PREFIX_PATH, which
should contain the install location of input packages. So the code proposed
in a previous message:
  (search-path-specification
    (variable "CMAKE_PREFIX_PATH")
    (directories '("")))
should work. In practice, in a user profile, it would amount to only the
profile root, which makes sense: This is the install location from a user
point of view. In the build environment, it would add one entry for each
input, but I do not see a way of singling out only the packages built with
cmake.

>    2. Add custom code to cmake-build-system.scm instead of using the
>       search path mechanism.

One could try to emulate the "trivial" search path above by adding each input
to CMAKE_PREFIX_PATH. But it would not solve the problem that a user install-
ing a library built with cmake (like libqtxdg) into his profile would not be
able to compile another package (like liblxqt) requiring it.

I also thought about searching for package-config.cmake files and defining
an environment variable PACKAGE_DIR if one is found. The problem is
capitalisation: Here we find
   /gnu/store/...-libqtxdg-1.1.0/share/cmake/qt5xdg/qt5xdg-config.cmake
and need to set the environment variable
   Qt5Xdg_DIR .
It is impossible to guess this!

All in all, I am in favour of using the trivial search path with the ""
directory. It would be activated only if cmake is an input, or a user
installs cmake into his profile.

Andreas