The iface_test_bld gets the excludedFromAll include directory with
"-isystem" because it is added indirectly through an imported target.
On AIX with GCC the -isystem flag causes sources to be preprocessed as:
# 3 "/.../excludedFromAll.h" 2 3 4
The flags after the file name are documented here:
https://gcc.gnu.org/onlinedocs/cpp/Preprocessor-Output.html
and the "4" flag says that following content is extern "C". This
causes the excludedFromAll function to be declared as a C symbol
and not mangled for C++, which fails to link later because the
symbol is really provided as C++.
Work around this by setting the NO_SYSTEM_FROM_IMPORTED target property
on iface_test_bld. Somehow iface_test_exp does not end up with -isystem
so we do not need this workaround for that target.
4fc4742477