autoconf detecting where pthread functions really are - pthreads

Question

I'm looking for a way to detect where pthread functions really exist (libc, libpthread or both). My motivation is that GNU libc seems to be moving the pthread functions from libpthread to libc. I need to intercept these functions and to determine which library to intercept at configure time. To complicate the supported platforms are Linux glibc, Solaris, FreeBSD, macOS and Linux musl, with the emphasis on Linux glibc.
I don't think that AC_CHECK_LIB and AC_SEARCH_LIBS are suitable, they will just try to compile a little stub containing the function and linking with the library. Since libc gets linked by default this won't work.
So far I've come up with something like this
safe_CFLAGS=$CFLAGS
CFLAGS="$CFLAGS -nodefaultlibs -lc"
AC_MSG_CHECKING([if pthread_cond_destroy is in libc])
AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[
#include <pthread.h>
(void)pthread_cond_destroy(0);
]])], [
PTHREAD_IN_LIBC=1
AC_MSG_RESULT([yes])
], [
AC_MSG_RESULT([no])
])
CFLAGS=$safe_CFLAGS
This seems to work OK-ish for detecting if functions are in libc (and perhaps AC_SEARCH_LIBS could do the same more easily).
But for checking libpthread I get
configure:16532: clang -o conftest -nodefaultlibs -lpthread
conftest.c >&5 ld: error: undefined symbol: atexit
>>> referenced by crt1.c:63 (/usr/src/lib/csu/amd64/crt1.c:63)
(plus similar errors for _init_tls and exit)
Plan B to use nm but that seems really brittle - libs may be stripped, and I'd have to handle every different path / library name.
UPDATE:
I've made some progress.
ac_pthread_create_in_libc=yes
ac_pthread_cond_destroy_in_libc=yes
ac_sem_open_in_libc=yes
AC_SEARCH_LIBS([pthread_create], , , [ac_pthread_create_in_libc=no],)
AC_SEARCH_LIBS([pthread_cond_destroy], , , [ac_pthread_cond_destroy_in_libc=no],)
AC_SEARCH_LIBS([sem_open], , , [ac_sem_open_in_libc=no],)
ac_thread_fns_only_in_libc=no
if test x$ac_pthread_create_in_libc = xyes \
-a x$ac_pthread_cond_destroy_in_libc = xyes \
-a x$ac_sem_open_in_libc = xyes ; then
ac_thread_fns_only_in_libc=yes
fi
ac_thread_fns_only_in_libpthread=no
if test x$ac_thread_fns_only_in_libc = xno ; then
ac_pthread_create_in_libpthread=no
ac_pthread_cond_destroy_in_libpthread=no
ac_sem_open_in_libpthread=no
if test x$ac_pthread_create_in_libc = xno ; then
unset ac_cv_search_pthread_create
safe_LIBS=$LIBS
AC_SEARCH_LIBS([pthread_create], [pthread], [ac_pthread_create_in_libpthread=yes], ,)
LIBS=$safe_LIBS
fi
if test x$ac_pthread_cond_destroy_in_libc = xno ; then
unset ac_cv_search_pthread_cond_destroy
AC_SEARCH_LIBS([pthread_cond_destroy], [pthread], [ac_pthread_cond_destroy_in_libc=yes], ,)
LIBS=$safe_LIBS
fi
if test x$ac_sem_open_in_libc= xyes ; then
unset ac_cv_search_sem_open
AC_SEARCH_LIBS([sem_open], [pthread], [ac_sem_open_in_libc=yes], ,)
LIBS=$safe_LIBS
fi
if test x$ac_pthread_create_in_libpthread = xyes \
-a x$ac_pthread_cond_destroy_in_libpthread = xyes \
-a x$ac_sem_open_in_libpthread = xyes ; then
ac_thread_fns_only_in_libpthread=yes
fi
fi
ac_thread_fns_in_libc_and_libpthread=no
if test x$ac_thread_fns_only_in_libc = xno -a \
x$ac_thread_fns_only_in_libpthread = xno ; then
ac_thread_fns_in_libc_and_libpthread=yes
fi
AC_MSG_NOTICE([libc $ac_thread_fns_only_in_libc pthread $ac_thread_fns_only_in_libpthread both $ac_thread_fns_in_libc_and_libpthread])
This seems to work. I'll make it an answer when it's production ready.