If `CMAKE_SYSROOT` is set by the user (e.g. via the toolchain file) then
we do not set `CMAKE_SYSROOT_COMPILE` to our default.  Fix our
references to the sysroot's `/usr/include` directory to use
`CMAKE_SYSROOT_COMPILE` only if it is set and else `CMAKE_SYSROOT`.

Fixes: #17096