WITH_/WITHOUT_CXX: update description

Contrary to the previous description WITHOUT_CXX does not disable
/usr/bin/c++, which is just a link to Clang.  We also no longer have
gperf.

MFC after:	1 week
Sponsored by:	The FreeBSD Foundation

tools/build/options/WITHOUT_CXX

 .\" $FreeBSD$
-Do not build
-.Xr c++ 1
-and related libraries.
-It will also prevent building of
-.Xr gperf 1
-and
+Do not build C++ headers and runtime libraries.
+It also prevents building binaries and libraries written in C++, including
 .Xr devd 8 .

tools/build/options/WITH_CXX

 .\" $FreeBSD$
-Build
-.Xr c++ 1
-and related libraries.
+Build C++ headers and runtime libraries.