Better support for thread-support flag detection with clang

When testing the stderr produced by various thread-support flags, also
run a compilation in addition to a link, because clang warns on
certain flags when compiling but not when linking.

config/acx_pthread.m4

@@ -142,7 +142,8 @@ main (int argc, char **argv)
 }
 _ACEOF
             rm -f conftest.$ac_objext conftest$ac_exeext
-            if test "`(eval $ac_link 2>&1 1>&5)`" = ""; then
+            # Check both linking and compiling, because they might tolerate different options.
+            if test "`(eval $ac_link 2>&1 1>&5)`" = "" && test "`(eval $ac_compile 2>&1 1>&5)`" = ""; then
                 # we continue with more flags because Linux needs -lpthread
                 # for libpq builds on PostgreSQL.  The test above only
                 # tests for building binaries, not shared libraries.

configure

@@ -22814,7 +22814,8 @@ main (int argc, char **argv)
 }
 _ACEOF
             rm -f conftest.$ac_objext conftest$ac_exeext
-            if test "`(eval $ac_link 2>&1 1>&5)`" = ""; then
+            # Check both linking and compiling, because they might tolerate different options.
+            if test "`(eval $ac_link 2>&1 1>&5)`" = "" && test "`(eval $ac_compile 2>&1 1>&5)`" = ""; then
                 # we continue with more flags because Linux needs -lpthread
                 # for libpq builds on PostgreSQL.  The test above only
                 # tests for building binaries, not shared libraries.