readline: fix build if ncurses headers are not found for various reasons

Signed-off-by: Stephan Raue <stephan@openelec.tv>
parted:init: fix build

Signed-off-by: Stephan Raue <stephan@openelec.tv>
config/functions: unset $CPP if set on host OS

Signed-off-by: Stephan Raue <stephan@openelec.tv>
Revert "readline: fix build if ncurses headers are not found for various reasons"

This reverts commit 70016a4c239c3d9b7325c98f01e3f97dce49bfbd.
Revert "llvm: dont check for gcc version when building hosttools, this should fix #3492"

This reverts commit 14d7610b72e1f54bc72fc3388cbe6ca71029c7e2.