Improve matio workaround for RHEL 6

Previous workaround was crashing the Windows snapshot

m4/ax_matio.m4

@@ -41,8 +41,10 @@ AC_ARG_WITH(matio, AC_HELP_STRING([--with-matio=DIR], [prefix to MATIO installat
   LDFLAGS="$LDFLAGS_MATIO $LDFLAGS"
 
   AC_CHECK_HEADER([matio.h], [], [has_matio=no])
-  dnl libz is explicitly added, as a workaround for the matio from EPEL 6 (used with RHEL 6)
-  AC_CHECK_LIB([matio], [Mat_Open], [LIBADD_MATIO="-lmatio -lz"], [has_matio=no], [-lz])
+  AC_CHECK_LIB([matio], [Mat_Open], [LIBADD_MATIO="-lmatio"], [
+    dnl Retest with libz explicitly added, as a workaround for the matio from RHEL 6 + EPEL 6
+    AC_CHECK_LIB([matio], [Mat_Open], [LIBADD_MATIO="-lmatio -lz"], [has_matio=no], [-lz])
+  ])
 
   CPPFLAGS="$ac_save_CPPFLAGS"
   LDFLAGS="$ac_save_LDFLAGS"