the _CS_V6_ENV and _CS_V7_ENV constants are required to be available for use
with confstr. glibc defines these constants with values 1148 and 1149,
respectively.

the only missing (and required) confstr constants are
_CS_POSIX_V7_THREADS_CFLAGS and _CS_POSIX_V7_THREADS_LDFLAGS which remain
unavailable in glibc.