Drop the config.h include from helpers.h after a report from a colleague
trying to build an external plugin from the latest GIT sources.
Instead, make sure config.h is included, and properly commented, in all
.c files that have configure #ifdefs and other deps. Also, move more
ot the includes from helpers.h to their respective .c file instead to
reduce the amount of headers an external plugin pulls in.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>