Add PG_TEST_EXTRA configure option to the Make builds

The Meson builds have PG_TEST_EXTRA as a configure-time variable,
which was not available in the Make builds. To ensure both build
systems are in sync, PG_TEST_EXTRA is now added as a configure-time
variable. It can be set like this:

    ./configure PG_TEST_EXTRA="kerberos, ssl, ..."

Note that to preserve the old behavior, this configure-time variable
is overridden by the PG_TEST_EXTRA environment variable when you run
the tests.

Author: Jacob Champion
Reviewed by: Ashutosh Bapat, Nazir Bilal Yavuz

configure

@@ -764,6 +764,7 @@ LDFLAGS
 CFLAGS
 CC
 enable_injection_points
+PG_TEST_EXTRA
 enable_tap_tests
 enable_dtrace
 DTRACEFLAGS
@@ -880,6 +881,7 @@ enable_largefile
       ac_precious_vars='build_alias
 host_alias
 target_alias
+PG_TEST_EXTRA
 CC
 CFLAGS
 LDFLAGS
@@ -1587,6 +1589,9 @@ Optional Packages:
   --with-openssl          obsolete spelling of --with-ssl=openssl
 
 Some influential environment variables:
+  PG_TEST_EXTRA
+              enable selected extra tests (overridden at runtime by
+              PG_TEST_EXTRA environment variable)
   CC          C compiler command
   CFLAGS      C compiler flags
   LDFLAGS     linker flags, e.g. -L<lib dir> if you have libraries in a
@@ -3629,6 +3634,7 @@ fi
 
 
 
+
 #
 # Injection points
 #