Improve compatibility/usability of using interop as a bundled shared library

[torbjoernk]

When integrating Interop as a dependency into another CMake-driven C++ project, it is difficult to govern the parts of Interop that are being installed/deployed together with the parent project.

The current approach I'm following consists of pulling in this Git repo as a submodule into $MY_PROJECT/external/interop and including it via add_subdirectory(interop) from within $MY_PROJECT/external/CMakeLists.txt. Thereby, I pre-set several Interop CMake options via set(... ON|OFF CACHE BOOL "" FORCE), e.g. set(ENABLE_CSHARP OFF CACHE BOOL "" FORCE) before the add_subdirectry(...) statement.

Issue 1
For my deployment scenario (Unix only) I'm preferring shared libraries. However, I don't see any possibility to select only one of libinterop_fpic_lib.so or libinterop_lib.so of ending up in $PREFIX/lib64 after a make install of my project.

Suggestion 1
add an option to choose between the position independent and the position dependent variants of libinterop while defaulting to the current behavior:

option(ENABLE_SHARED_FPIC "build (and install) a position-independent variant of the share library" ON)
option(ENABLE_SHARED_FPIC_ONLY "only build (and install) the position-independent variant of the shared library" OFF)

Resulting in the following combination matrix:

ENABLE_SHARED_FPIC / ENABLE_SHARED_FPIC_ONLY	libinterop_fpic_lib.so	libinterop_lib.so
ON / OFF	X	X
ON / ON	X	-
OFF / any	-	X

Issue 2
In addition, the LIBRARY DESTINATION and ARCHIVE DESTINATION paths are hard coded to lib64. It would be great utilizing GNUInstallDirs's LIBDIR to get a consistent path across a CMake (super-)project on the same system.

Suggestion 2
Use GNUInstallDirs's LIBDIR/BINDIR when defining the destination of LIBRARY and ARCHIVE install targets:

include(GNUInstallDirs)
install(TARGETS ${INTEROP_LIB}
        LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
        RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
        ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})

[ezralanglois]

Thank you for the suggestions. We just started to refactor the build scripts, so we can incorporate these in the next release.

I don't have a workaround for Issue 1: building both libraries. In the future, we won't build a position independent library when building shared libraries.

For Issue 2, you could use ExternalProject_Add and point it to the git repo. You can then override the installation behavior in a number of ways to achieve what you want above. For example you could use:

set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_INSTALL_LIBDIR})
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_INSTALL_LIBDIR})
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_INSTALL_BINDIR})

And skip the install step, so the build puts the libraries where you want them.

I don't know much about GNUInstallDirs, but we can look into using in the future.