build: avoid link failure on Windows

We only know -lpthread exists on platforms where we build
threads-pthread.c; but when we build threads-win32.c, LIB_PTHREAD
is empty.

* tests/Makefile.am (shunloadtest_LDADD): Use correct library.

tests/Makefile.am

@@ -530,7 +530,7 @@ libshunload_la_LDFLAGS = -module -avoid-version -rpath /evil/libtool/hack/to/for
 
 shunloadtest_SOURCES = \
 	shunloadtest.c
-shunloadtest_LDADD = -lpthread
+shunloadtest_LDADD = $(LIB_PTHREAD)
 shunloadtest_DEPENDENCIES = libshunload.la
 
 if WITH_CIL