[cmake-developers] -isystem suppresses warnings from user header

[Tamás Kenéz]

Environment: CMake 3.5, linux, g++

CMake uses "-isystem" on the compiler command line for the
INTERFACE_INCLUDE_DIRECTORIES of imported libraries.

Unfortunately including a header via an isystem path suppresses warnings
emitted from the header, such as warnings caused by expansions of macros
defined in that header.

Example: I have a printf-like LOG(...) macro which expands to a function
marked as printf-like. The -Wformat warnings are suppressed if the
containing log.h is found on isystem path.

Question: is this the intended behaviour? Can I force the -I flag somehow
for my library? (I'm aware of CMAKE_INCLUDE_SYSTEM_FLAG_CXX but I don't
want to modify all the client projects that uses my library.)

Or is it bug?

Thanks,
Tamas
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