cmake: Use LIB_INSTALL_DIR instead of hardcoding lib destination

[vadorovsky]

The comon practice in cmake to allow to specify the libdir is
using the LIB_INSTALL_DIR variable. The main use case of that
variable is setting /usr/lib64 as the destination dir for
libraries which is a common pattern in many Linux distributions.

[rnburn]

👍

[mdouaihy]

Hello,

this change breaks the build of OpenTracing dependency with Hunter.

CMake Error at /Users/mehrezdouaihy/.hunter/_Base/d93ace4/dee24f9/0d4bf3d/Install/lib/cmake/OpenTracing/OpenTracingTargets.cmake:80 (message):
  The imported target "OpenTracing::opentracing-static" references the file

     "/Users/mehrezdouaihy/.hunter/_Base/d93ace4/dee24f9/0d4bf3d/Build/opentracing-cpp/Install/lib/libopentracingd.a"

  but this file does not exist.  Possible reasons include:

In fact, Hunter expects the generated targets CMake files to handle moving them (from Build to its Install directory). This is done by CMake if the destination is not an absolute path.

With this change, the destination is absolute, CMake generates files that are incompatible with Hunter conventions.

You can check the following Hunter packaging

• with the change, not working: https://github.com/mdouaihy/hunter/archive/v0.21.7.tar.gz
• with the change, working: reverted https://github.com/mdouaihy/hunter/archive/v0.21.8.tar.gz

[rnburn]

Would changing

if (NOT DEFINED LIB_INSTALL_DIR)
    set(LIB_INSTALL_DIR ${CMAKE_INSTALL_PREFIX}/lib)
endif()

to

if (NOT DEFINED LIB_INSTALL_DIR)
    set(LIB_INSTALL_DIR lib)
endif()

fix things?

[mdouaihy]

Yes. I did the change and issued a PR.

[vadorovsky]

@mdouaihy Thanks and sorry for breaking Hunter.

[mdouaihy]

@mrostecki no worries.