[fix] ensure we fail if pthreads are not available

One more silent error. ax_pthreads will fail on -Werror, so we still
need to fix that.

configure.ac

@@ -67,7 +67,7 @@ AC_CHECK_LIB(dl, dlopen)
 
 # add pthread support.
 # doc in m4/ax_pthread.m4. Defines automake PTHREAD_CFLAGS and PTHREAD_LIBS
-AX_PTHREAD
+AX_PTHREAD([],[AC_MSG_ERROR([Cannot find how to compile with pthreads.])])
 CC="$PTHREAD_CC"
 
 # NUMA support

tests/Makefile.am

 AM_COLOR_TESTS = yes
 
-# add OpenMP to flags for test programs
-AM_CFLAGS = -I$(top_srcdir)/include
-AM_LDFLAGS = ../src/libaml.la
+AM_CFLAGS = -I$(top_srcdir)/include $(PTHREAD_CFLAGS)
+AM_LDFLAGS = ../src/libaml.la $(PTHREAD_LIBS)
 
 # valgrind support
 if TEST_VALGRIND