style.Makefile(5): mention C++ variables and LIBADD

[ngie]

Summary

This change mentions several C++-related build variables, e.g.,
CXXFLAGS for explicitess (even though it is generally implied
wherever CFLAGS is used). In order to make some of the text more
generic, an * is added between C and FLAGS, for brevity.

This change also mentions LIBADD: the current recommended interface
for accounting for library dependencies in-tree.

MFC after: 1 week

[brooks]

In general I think it would be better to expand

.Va CFLAGS

to

.Va CFLAGS
or
.Va CXXFLAGS

Except where they are used in the same or adjacent sentences so preserve searchability.

share/man/man5/style.Makefile.5

136

This pair of sentences is rather redundant and could be rewritten as something much shorter with fewer mentions of CFLAGS. Maybe something like

.Va WARNS
comes before
.Va CFLAGS
and
.Va CXXFLAGS ,
as it modifies
.Va C*FLAGS and should not be lost in a sea of additions.