Do not enforce POSIX if __STRICT_ANSI__ is defined

On b4c1548 I proposed defining _POSIX_C_SOURCE in
order to enable the macros to use POSIX under unix-like
systems, as POSIX was used anyway.

Here I propose to follow whatever the user wants. This means:

- If the user explicitly defines a strict standard C compliance
  (e.g. compiling using `--std=c99`) then we do not use
  any POSIX feature. This is detected through the definition
  of the __STRICT_ANSI__ macro.

- If the user does not enforce any C standard, we assume POSIX
  is available as before if the system is unix-like.

I believe this approach is better than the previous one because if
any cutest user does not want to rely on POSIX features we do not
force it. However by default we use them if they are available.

include/cutest.h

@@ -87,13 +87,12 @@
  **********************/
 
 /* The unit test files should not rely on anything below. */
-
 #include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
-#if defined(unix) || defined(__unix__) || defined(__unix) || defined(__APPLE__)
+#if (defined(unix) || defined(__unix__) || defined(__unix) || defined(__APPLE__)) && !defined(__STRICT_ANSI__)
     #define CUTEST_UNIX__    1
     #include <errno.h>
     #include <unistd.h>
@@ -102,12 +101,16 @@
     #include <signal.h>
 #endif
 
-#if defined(_WIN32) || defined(__WIN32__) || defined(__WINDOWS__)
+#if (defined(_WIN32) || defined(__WIN32__) || defined(__WINDOWS__)) && !defined(__STRICT_ANSI__)
     #define CUTEST_WIN__     1
     #include <windows.h>
     #include <io.h>
 #endif
 
+#if defined(__STRICT_ANSI__)
+    #define CUTEST_ANSI__    1
+#endif
+
 #ifdef __cplusplus
     #include <exception>
 #endif
@@ -352,9 +355,10 @@ test_do_run__(const struct test__* test)
     return (test_current_failures__ == 0) ? 0 : -1;
 }
 
+#if !defined(CUTEST_ANSI__)
 /* Called if anything goes bad in cutest, or if the unit test ends in other
  * way then by normal returning from its function (e.g. exception or some
- * abnormal child process termination). */
+ * abnormal child process termination). On CUTEST_ANSI__ this is not used so we don't define it*/
 static void
 test_error__(const char* fmt, ...)
 {
@@ -377,6 +381,7 @@ test_error__(const char* fmt, ...)
         printf("\n");
     }
 }
+#endif
 
 /* Trigger the unit test. If possible (and not suppressed) it starts a child
  * process who calls test_do_run__(), otherwise it calls test_do_run__()
@@ -458,7 +463,7 @@ test_run__(const struct test__* test)
             failed = 1;
         }
 
-#else
+#else /* CUTEST_ANSI__ */
 
         /* A platform where we don't know how to run child process. */
         failed = (test_do_run__(test) != 0);