check for malloc before modifying LIBS

the configure-time checks will get confused on some systems if we
populate LIBS and CPPFLAGS with our mochi service dependencies.  Thanks
for the bug report, Chuck Cranor.

configure.ac

@@ -28,13 +28,6 @@ if test "x$PKG_CONFIG" == "x"; then
    AC_MSG_ERROR([Could not find pkg-config utility!])
 fi
 
-PKG_CHECK_MODULES([MARGO],[margo],[],
-      AC_MSG_ERROR([Could not find working margo installation!]) )
-LIBS="$MARGO_LIBS $LIBS"
-CPPFLAGS="$MARGO_CFLAGS $CPPFLAGS"
-CFLAGS="$MARGO_CFLAGS $CFLAGS"
-
-
 # Checks for typedefs, structures, and compiler characteristics.
 AC_C_INLINE
 AC_TYPE_INT64_T
@@ -45,6 +38,12 @@ AC_TYPE_SSIZE_T
 AC_FUNC_MALLOC
 AC_FUNC_REALLOC
 
+PKG_CHECK_MODULES([MARGO],[margo],[],
+      AC_MSG_ERROR([Could not find working margo installation!]) )
+LIBS="$MARGO_LIBS $LIBS"
+CPPFLAGS="$MARGO_CFLAGS $CPPFLAGS"
+CFLAGS="$MARGO_CFLAGS $CFLAGS"
+
 
 bwtree_backend=yes
 AC_ARG_ENABLE([bwtree],