[build] Error exposing HDF5 to user codes with cmake 3.12

[mwallerb]

It's the next round of CMake's favourite game: Let's Break User Space!^(TM)

CMake 3.12 made some changes that stops ALPSCore from exposing its HDF5 dependency to user codes. Compiling a user codes results in the following error message:

CMake Error at CMakeLists.txt:17 (add_executable):
  Target "var1-bin" links to target "hdf5::hdf5-shared" but the target was
  not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?

As a workaround, one has to explicitly declare the HDF5 dependency before ALPSCore dependency:

find_package(HDF5 REQUIRED)
find_package(ALPSCore REQUIRED)

[egull]

Crap. @galexv please have a look and prioritize this!

[galexv]

@mwallerb Cannot reproduce it with CMake 3.12.2 and HDF5 1.8.16 under Ubuntu 16.04.
Could you please give more info: OS, version, HDF5 version... (and possibly send me CMakeCache.txt of both ALPSCore and the client code)?

[mwallerb]

Arch Linux
CMake 3.12.2
HDF5 1.10.3

Cache files per mail...

[galexv]

Thanks @mwallerb. I have reproduced the issue.
It arises when CMake uses hdf5-config.cmake which is supplied with HDF5-1.10 when it is installed via CMake. Properly handling it needs some changes to ALPSCore cmake scripts, but a quick workaround will be pushed shortly.

[galexv]

(I am not closing yet: we should still handle targets imported from external libraries properly)