c++ - Conditional transitive link libraries -

i'm struggling cmake "config"-based package description when using 2 depending packages upstream package has optional link libraries.

suppose have liba optionally uses zip functionality (say option use_zlib in liba's cmakelists.txt). means liba optionally have -lz in interface_link_libraries (exported liba-config.cmake). now, if have libb using liba via find_package(liba), how know if liba has been built or without zlib support when both libraries installed , exported cmake's package config system?

i know link library information treated transitively , propagated link libraries of libb, hence -lz appear whenever linking against libb. however, path libz included; consequently, link fails "could not find library z" unless magically knew libz link library of liba , include from. including absolute path libz in liba's config not way go, ruins portability on different systems.

edit: i've been getting replies suggesting use target_link_libraries(liba private z), e.g. declaring (in fact internal liba) libz private library. unfortunately not solve issue, cmake automatically adds private-marked link library interface_link_libraries list via $<link_only:z>. shows necessity of having -lz appear in exported link library list in way (and cmake export script authors understand transitive issue).

the canonical way making liba-config.cmake template , configure_file() populate information build options.

for example, cmakelists.txt looks this:

if(zlib_found) ... # link zlib set(built_with_zlib 1) endif() ... # more options configure_file(alib-config.cmake.in alib-config.cmake) 

now, liba-config.cmake.in should have

set(built_with_zlib @built_with_zlib@) if(built_wth_zlib) # append lz alib_libraries or whetever variable called endif() 

after configuration alib-config.cmake contain set(built_with_zlib 0) or set(built_with_zlib 1) depending on value in cmake.