Bug#45288: pb2 returns a lot of compilation warnings

Fix assorted warnings that are generated in optimized builds.
Most of it is silencing variables that are set but unused.

This patch also introduces the MY_ASSERT_UNREACHABLE macro
which helps the compiler to deduce that a certain piece of
code is unreachable.

include/my_compiler.h:
  Use GCC's __builtin_unreachable if available. It allows
  GCC to deduce the unreachability of certain code paths,
  thus avoiding warnings that, for example, accused that a
  variable could be used without being initialized (due to
  unreachable code paths).

include/my_compiler.h

@@ -32,8 +32,15 @@
 
 /* GNU C/C++ */
 #if defined __GNUC__
+/* Convenience macro to test the minimum required GCC version. */
+# define MY_GNUC_PREREQ(maj, min) \
+    ((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
 /* Any after 2.95... */
 # define MY_ALIGN_EXT
+/* Comunicate to the compiler the unreachability of the code. */
+# if MY_GNUC_PREREQ(4,5)
+#   define MY_ASSERT_UNREACHABLE()   __builtin_unreachable()
+# endif
 
 /* Microsoft Visual C++ */
 #elif defined _MSC_VER
@@ -67,7 +74,7 @@
 #endif
 
 /**
-  Generic compiler-dependent features.
+  Generic (compiler-independent) features.
 */
 #ifndef MY_ALIGNOF
 # ifdef __cplusplus
@@ -79,6 +86,10 @@
 # endif
 #endif
 
+#ifndef MY_ASSERT_UNREACHABLE
+# define MY_ASSERT_UNREACHABLE()  do { assert(0); } while (0)
+#endif
+
 /**
   C++ Type Traits
 */