Building libint2.6.0-beta6 as follows on MacOS:
(Same issue with AppleClang and brew installed gcc)

cmake -DCMAKE_CXX_FLAGS_INIT="-march=native -Wno-unused-variable" -DCMAKE_BUILD_TYPE=Release -GNinja

ninja -j12

[4217/4217] Linking CXX static library libint2.a
FAILED: libint2.a
: && /usr/local/Cellar/cmake/3.14.5/bin/cmake -E remove libint2.a && /usr/bin/ar qc libint2.a @CMakeFiles/int2.rsp && /usr/bin/ranlib libint2.a && /usr/local/Cellar/cmake/3.14.5/bin/cmake -E touch libint2.a && :
ar: @CMakeFiles/int2.rsp: No such file or directory
ninja: build stopped: subcommand failed.