Hi,
I’m compiling against the C++ API on macOS using GCC-9.3, and cmake seems to use a bad flag:
... -fopenmp -D_GLIBCXX_USE_CXX11_ABI= -std=c++14 ... – note how it “blanks out” the _GLIBCXX_USE_CXX11_ABI variable. This causes the compiler to fail in the stdlib:
/usr/local/Cellar/gcc@9/9.3.0/include/c++/9.3.0/x86_64-apple-darwin18/bits/c++config.h:273:27: error: #if with no expression
  273 | #if _GLIBCXX_USE_CXX11_ABI

What’s probably going on here is that -D_GLIBCXX_USE_CXX11_ABI= (instead of -D_GLIBCXX_USE_CXX11_ABI=0 or -D_GLIBCXX_USE_CXX11_ABI=1) causes the compiler substitute nothing for _GLIBCXX_USE_CXX11_ABI, which means that the #if _GLIBCXX_USE_CXX11_ABI becomes #if  – what we would want is #if 0
Any advice on how to fix this would be much apreciated – I’ve tried manually setting _GLIBCXX_USE_CXX11_ABI, but it seems to be overwritten somewhere.
Cheers,
Johannes