CMake builds fails on QNX (qcc): Incorrect assembler invocation

[Felix-El]

While trying to build Boost for QNX with CMake, I faced the issue that -A,--defsym assmbler flags were not accepted by qcc.
Only when I replaced all defines without assignment.

target_compile_definitions(boost_context VAR)

into

target_compile_definitions(boost_context VAR=)

would the build succeed. I did not face the issue while building with b2.
Would it be possible to either really apply this approach and/or watch more closely which definitions are passed to the assembler?