winpthreads: make testing shared library work

Running `make check` when configured with `--disable-static`,
depending on whether an installed winpthread library can be found
when linking tests, has the following consequences:

If an installed winpthread library is found, tests will be linked
against installed library instead of the one we just built.

If there is no installed winpthread library, tests will be linked
against shared library. This will result in tests failing, since
they will fail to locate just built DLL at runtime.

To fix this:

Remove `-static` from `AM_LDFLAGS`, so we can link against the DLL.

Use LDADD to link tests against just built winpthread library.
Using `-l` is inappropriate to link against built libraries as it
should only be used for preinstalled libraries. Instead, pass
winpthread.la through LDADD.

Executable wrappers created by libtool now will handle runtime
search path.

Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>

mingw-w64-libraries/winpthreads/tests/Makefile.am

diff --git a/mingw-w64-libraries/winpthreads/tests/Makefile.am b/mingw-w64-libraries/winpthreads/tests/Makefile.am
index 9dda82f..7fc84d0 100644
--- a/mingw-w64-libraries/winpthreads/tests/Makefile.am
+++ b/mingw-w64-libraries/winpthreads/tests/Makefile.am
@@ -1,13 +1,12 @@
 AM_CPPFLAGS = -I$(top_srcdir)/src -I$(top_srcdir)/include
-AM_LDFLAGS =
+AM_LDFLAGS = -L$(top_builddir)
 
 if !MSVC
 AM_LDFLAGS += -L$(top_builddir)/fakelib
 endif
 
-AM_LDFLAGS += -L$(top_builddir) -lwinpthread -static
+LDADD = $(top_builddir)/libwinpthread.la
 
 #FIXME: The test "test.c" is inherently broken currently.
 check_PROGRAMS = t_clock_getres  t_clock_gettime  t_clock_nanosleep  t_clock_settime  t_nanosleep  #test
 TESTS = $(check_PROGRAMS)
-