mqtt build fails since resolution of #399

[ropieur]

cmake ends up with the following error

CMake Error at include/CMakeLists.txt:56 (string):
string sub-command REPLACE requires at least four arguments.

Might be $ENV{CXXFLAGS} is empty?

[ropieur]

I confirm that the fix works for me.
Shouldn't you consider the same kind of fix for the previous line in CMakeLists.txt?

[redboltz]

Thanks. At first, I thought that CMAKE_CXX_FLAGS is set at root of CMakeLists.txt. But I found the path that CMAKE_CXX_FLAGS isn't set. So I updated the PR.