cmake: skip build system check on compile

cmake checks the build system and its variables on its own to detect if
the makefiles need to be regenerated.
Unfortunately this can invalidate overrides passed in the
Build/Configure step. On non-Linux systems this breaks the build when
switching between targets of the same package architecture.

Fix this by forcibly disabling the build system check and relying on the
LEDE build system to take care of these things

Signed-off-by: Felix Fietkau <nbd@nbd.name>