Add a directory for tests that exercise the supported C standard differences.

As a first test added the snippet given in issue #1670

test/standard/bug1670.c

@@ -0,0 +1,33 @@
+
+/* #1670 - Standard headers contain non standard identifiers in C89/C99 mode */
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <time.h>
+
+#if __CC65_STD__ != __CC65_STD_CC65__
+/* implement our own clock_gettime, using a different signature than the POSIX one */
+const char* clock_gettime(void)
+{
+    static char buf[32];
+    struct tm *my_tm;
+#if 0
+    /* FIXME: this will not work in the simulator */
+    time_t t = time(NULL);
+#else
+    time_t t = 0x12345678;
+#endif
+    my_tm = localtime(&t);
+    printf("%2d:%2d:%2d\n", my_tm->tm_hour, my_tm->tm_min, my_tm->tm_sec);
+    strftime(buf, sizeof(buf), "<%H:%M:%S>", my_tm);
+    return buf;
+}
+#endif
+
+int main(void)
+{
+#if __CC65_STD__ != __CC65_STD_CC65__
+    printf("The time is %s\n", clock_gettime());
+#endif
+    return EXIT_SUCCESS;
+}