pgsql: oauth: Run Autoconf tests with correct compiler flags

oauth: Run Autoconf tests with correct compiler flags

Commit b0635bfda split off the CPPFLAGS/LDFLAGS/LDLIBS for libcurl into
their own separate Makefile variables, but I neglected to move the
existing AC_CHECKs for Curl into a place where they would make use of
those variables. They instead tested the system libcurl, which 1) is
unhelpful if a different Curl is being used for the build and 2) will
fail the build entirely if no system libcurl exists. Correct the order
of operations here.

Reported-by: Ivan Kush <ivan(dot)kush(at)tantorlabs(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Ivan Kush <ivan(dot)kush(at)tantorlabs(dot)com>
Discussion: https://postgr.es/m/8a611028-51a1-408c-b592-832e2e6e1fc9%40tantorlabs.com
Backpatch-through: 18

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/3d23f68c5529b5a38c6f38ebd8fae518b5a369d0

Modified Files
--------------
config/programs.m4 | 18 ++++++++++++------
configure | 18 ++++++++++++------
2 files changed, 24 insertions(+), 12 deletions(-)