configure, drd: Only build the swapcontext test if swapcontext() is available

Add a configure test for swapcontext() since MUSL does not provide a
swapcontext() implementation. See also
https://bugs.kde.org/show_bug.cgi?id=434775 .

configure.ac

@@ -4403,6 +4403,7 @@ AC_CHECK_FUNCS([     \
         strpbrk      \
         strrchr      \
         strstr       \
+        swapcontext  \
         syscall      \
         utimensat    \
         process_vm_readv  \
@@ -4433,6 +4434,7 @@ AM_CONDITIONAL([HAVE_PREADV_PWRITEV],
                [test x$ac_cv_func_preadv = xyes && test x$ac_cv_func_pwritev = xyes])
 AM_CONDITIONAL([HAVE_PREADV2_PWRITEV2],
                [test x$ac_cv_func_preadv2 = xyes && test x$ac_cv_func_pwritev2 = xyes])
+AM_CONDITIONAL([HAVE_SWAPCONTEXT], [test x$ac_cv_func_swapcontext = xyes])
 AM_CONDITIONAL([HAVE_MEMFD_CREATE],
                [test x$ac_cv_func_memfd_create = xyes])
 

drd/tests/Makefile.am

@@ -408,7 +408,6 @@ check_PROGRAMS =      \
   sem_as_mutex        \
   sem_open            \
   sigalrm             \
-  swapcontext         \
   threaded-fork       \
   trylock             \
   unit_bitmap         \
@@ -470,6 +469,10 @@ if HAVE_PTHREAD_SPINLOCK
 check_PROGRAMS += pth_spinlock
 endif
 
+if HAVE_SWAPCONTEXT
+check_PROGRAMS += swapcontext
+endif
+
 if !VGCONF_OS_IS_DARWIN
 check_PROGRAMS += sem_wait
 endif