configure.ac: require pthread-stubs only where available

The project is a thing only for BSD platforms. Or in other words - for
any other platforms building/installing pthread-stubs results only in a
pthread-stub.pc file.

And even where it provides a DSO, there's a fundamental design issue
with it - see the pthread-stubs mailing list for the specifics.

v2: Update comment above the switch statement (Jon Turney).

Reviewed-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Acked-by: Gary Wong <gtw@gnu.org>
Tested-by: Eric Engestrom <eric.engestrom@imgtec.com>
Acked-by: Randy Fishel <randy.fishel@oracle.com>
Cc: Niveditha Rau <niveditha.rau@oracle.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

configure.ac

@@ -801,9 +801,10 @@ if test "x$android" = xno; then
     test -z "$PTHREAD_LIBS" && PTHREAD_LIBS="-lpthread"
 fi
 
-dnl pthread-stubs is mandatory on targets where it exists
+dnl pthread-stubs is mandatory on BSD platforms, due to the nature of the
+dnl project. Even then there's a notable issue as described in the project README
 case "$host_os" in
-cygwin* )
+linux* | cygwin* | darwin* | solaris* | gnu*)
     pthread_stubs_possible="no"
     ;;
 * )