Check if struct stat has mtim member

Not all libc's have a mtim member in struct stat (dietlibc doesn't).
Change ts_usec() to receive a struct stat as parameter and implement it
accordingly for both cases.
diff --git a/configure.ac b/configure.ac
index 66c2d51..19f78d5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -79,6 +79,9 @@
 	AC_DEFINE(ENABLE_DEBUG, [1], [Debug messages.])
 ])
 
+# dietlibc doesn't have st.st_mtim struct member
+AC_CHECK_MEMBERS([struct stat.st_mtim], [], [], [#include <sys/stat.h>])
+
 CC_CHECK_CFLAGS_APPEND([ \
 			-pipe \
 			-DANOTHER_BRICK_IN_THE \
@@ -125,7 +128,6 @@
 			-Wl,--as-needed \
 			-Wl,--gc-sections])
 
-
 AC_CONFIG_HEADERS(config.h)
 AC_CONFIG_FILES([
 	Makefile