<wchar.h>: Partial revert of b9286c919de6585c967b8dbf2c472fba7b33f9f1.

<wchar.h>: Partial revert of b9286c919de6585c967b8dbf2c472fba7b33f9f1.

Before Hasso's commit, both wchar_t and WCHAR_{MIN,MAX} were inside a
#ifndef __cplusplus. With this state, devel/boost-libs in pkgsrc indeed
fails to build because it can't find WCHAR_{MIN,MAX}.

However, with the #ifndef completely removed (after Hasso's commit),
clang whines during C++ compilation because wchar_t is a built in type
in C++.

Therefore, bring back the #ifndef, but only around wchar_t and not
around the limits. This keeps boost-libs building, silences clang, is
in line with what the C++ standard says and with what others (FreeBSD,
for example) do.