mmprivate.h leaks (intentionally) macros with very common names.

Exporting macros in header files uncontrollably usually hurts. In this case,
Node.inc (from LLVM) tries to define a macro with the same name (BLOCK) but in
a function style way. This causes an error in module builds, we don't see this
in non-module builds because of the 'lucky' alignment of translation units.

This patch works around the bug by undefining the macro before entering the
headers that are redefining it, but we *really* should not expose mmprivate.h in
$ROOTSYS/include because it can alter the behavior of a lot of user code.