cmake 3.9 for android is broken
https://github.com/android-ndk/ndk/issues/467
-isystem /usr/include
is appended to compiler flags. clang will not prefix SYSROOT
for those dirs, instead -iwithsysroot /usr/include
should be used. gcc is a little different. According the document https://gcc.gnu.org/onlinedocs/gcc/Directory-Options.html
Add the directory dir to the list of directories to be searched for header files during preprocessing. If dir begins with ‘=’ or $SYSROOT, then the ‘=’ or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.
, -isystem=/usr/include
or -isystem =/usr/include
should be used.