Add UNCONSTIFY() macro.

Allows casting const-ness away from an expression, but doesn't allow changing the type. Enforcement of the latter currently only works for gcc-like compilers.

Note that it is not safe to cast const-ness away if the result will ever be modified (it would be undefined behavior). Doing so can cause compiler mis-optimizations or runtime crashes (by modifying read-only memory). It is only safe to use when the result will not be modified, but API design or language restrictions prevent you from declaring that (e.g. because a function returns both const and non-const variables).

Note that this only works in function scope, not for global variables (it would be nice, but not trivial, to improve that).

UNCONSTIFY() requires static assert which is a feature in its own right.

test/lib/pgBackRestTest/Common/JobTest.pm

@@ -438,6 +438,8 @@ sub run
 
                 # Create build.auto.h
                 my $strBuildAutoH =
+                    ($self->{oTest}->{&TEST_VM} ne VM_CO6 ? "#define HAVE_STATIC_ASSERT\n" : '') .
+                    "#define HAVE_BUILTIN_TYPES_COMPATIBLE_P\n" .
                     (vmWithLz4($self->{oTest}->{&TEST_VM}) ? '#define HAVE_LIBLZ4' : '') . "\n";
 
                 buildPutDiffers($self->{oStorageTest}, "$self->{strGCovPath}/" . BUILD_AUTO_H, $strBuildAutoH);