devel/gzstream: Link with ${CXX}

libgzstream is a C++ library so should be linked with libc++. This
should unbreak consumers like audio/pianod2 after base r345349.
Also use INSTALL_LIB to install the library and make the build
respect LDFLAGS.

ld: error: /usr/local/lib/libgzstream.so: undefined reference to std::1::basic_streambuf<char, std::1::char_traits<char> >::imbue(std::1::locale const&)
...
ld: error: /usr/local/lib/libgzstream.so: undefined reference to std::1::ios_base::init(void*)
ld: error: /usr/local/lib/libgzstream.so: undefined reference to std::1::basic_ios<char, std::1::char_traits<char> >::~basic_ios()
ld: error: /usr/local/lib/libgzstream.so: undefined reference to typeinfo for std::1::basic_streambuf<char, std::1::char_traits<char> >
ld: error: /usr/local/lib/libgzstream.so: undefined reference to typeinfo for std::1::basic_ios<char, std::1::char_traits<char> >
ld: error: /usr/local/lib/libgzstream.so: undefined reference to vtable for cxxabiv1::vmi_class_type_info

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p501607_s347555/logs/errors/pianod2-301.log