glibc: ensure BR2_MAKE is also used for subdirs

The configure script contains logic to figure out what make program to
invoke for subdirectories (trying gnumake, gmake, make).  Explicitly force
it to use our BR2_MAKE to ensure the right make version is used.

As GLIBC_CONF_ENV is only initialized below, move this section below it.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>