glibc: move autoconf cache variables to GLIBC_CONF_ENV

glibc: move autoconf cache variables to GLIBC_CONF_ENV

glibc is not using the autotools-package infrastructure, so we are
calling the ./configure script manually. Currently, the few autoconf
cache variables are passed as arguments to the ./configure script,
while we pass them through the environment in the autotools-package
infrastructure.

So let's pass them in the environment, and use a GLIBC_CONF_ENV
variable to store them. This will allow in a follow-up commit to
conditionally add more autoconf cache variables to the glibc build.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>