Makefile: move -DPAGER_ENV from BASIC_CFLAGS to EXTRA_CPPFLAGS

Makefile: move -DPAGER_ENV from BASIC_CFLAGS to EXTRA_CPPFLAGS

Remove -DPAGER_ENV from the BASIC_CFLAGS and instead have it passed
via the EXTRA_CPPFLAGS passed when compiling pager.c.

This doesn't change anything except to make it clear that only pager.c
needs this, as it's the only user of this define. See
995bc22d7f8 (pager: move pager-specific setup into the build,
2016-08-04) for the commit that originally added this.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>