helper,rtos,server: Restore missing warning flags

helper,rtos,server: Restore missing warning flags

These libraries override the used CFLAGS without adding the
defaults. This didn't have any effect until change
http://openocd.zylin.com/3870 (ef4c139). Restore by adding
AM_CLAGS to the per-target CFLAGS.

Interestingly, automake seems to clear the CFLAGS for the target
even if the override variable is only mentioned within a non-active
conditional branch, such as the IS_MINGW for the affected libraries.

Change-Id: I805206865e59e3fa33a7ea3c0d3472e51219351c
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/3927
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>