Hinge GCC #pragma diagnostics on version 4.6, not 4.2.

GCC release notes document all GCC diagnostic pragmas as new in GCC 4.6,
but this is not entirely true: in GCC 4.4.5 "push" and "pop" are indeed
not available, but "error", "warning" and "ignored" are (the warnings
stand for DIAG_OFF_DEPRECATION and DIAG_ON_DEPRECATION respectively):

tcpdump.c: At top level:
tcpdump.c:3451: warning: expected [error|warning|ignored] after '#pragma
  GCC diagnostic'
tcpdump.c:3480: warning: expected [error|warning|ignored] after '#pragma
  GCC diagnostic'

Before commit 39f09d6 in 2021 the code indeed required GCC 4.6 for
"push" and "pop" consistently with libpcap, but during the refactoring
the requirement erroneously became 4.2, so make it 4.6 again.

Author: infrastation

diag-control.h

@@ -132,7 +132,7 @@
     #define DIAG_ON_STRICT_PROTOTYPES \
       DIAG_DO_PRAGMA(clang diagnostic pop)
   #endif
-#elif ND_IS_AT_LEAST_GNUC_VERSION(4,2)
+#elif ND_IS_AT_LEAST_GNUC_VERSION(4,6)
   /* GCC apparently doesn't complain about ORing enums together. */
 
   /*
@@ -145,21 +145,19 @@
   #define DIAG_ON_CAST_QUAL \
     DIAG_DO_PRAGMA(GCC diagnostic pop)
 
-  #if ND_IS_AT_LEAST_GNUC_VERSION(4,5)
-    /*
-     * GCC warns about unused return values if a function is marked as
-     * "warn about ignoring this function's return value".
-     *
-     * Clang appears to let you ignore a result without a warning by
-     * casting the function result to void, so we don't appear to
-     * need this for Clang.
-     */
-    #define DIAG_OFF_WARN_UNUSED_RESULT \
-      DIAG_DO_PRAGMA(GCC diagnostic push) \
-      DIAG_DO_PRAGMA(GCC diagnostic ignored "-Wunused-result")
-    #define DIAG_ON_WARN_UNUSED_RESULT \
-      DIAG_DO_PRAGMA(GCC diagnostic pop)
-  #endif
+  /*
+   * GCC warns about unused return values if a function is marked as
+   * "warn about ignoring this function's return value".
+   *
+   * Clang appears to let you ignore a result without a warning by
+   * casting the function result to void, so we don't appear to
+   * need this for Clang.
+   */
+  #define DIAG_OFF_WARN_UNUSED_RESULT \
+    DIAG_DO_PRAGMA(GCC diagnostic push) \
+    DIAG_DO_PRAGMA(GCC diagnostic ignored "-Wunused-result")
+  #define DIAG_ON_WARN_UNUSED_RESULT \
+    DIAG_DO_PRAGMA(GCC diagnostic pop)
 
   /*
    * Suppress deprecation warnings.