devel/libhoard: Unbreak build with Clang 6 (C++14 by default)

include/hoard/geometricsizeclass.h:137:5: error: non-type template argument evaluates to -2147483648, which cannot be narrowed to type 'size_t' (aka 'unsigned long') [-Wc++11-narrowing]

MaxObjectSize>::VALUE };
^

http://beefy3.nyi.freebsd.org/data/112amd64-quarterly/491309/logs/errors/libhoard-3.10.log

• Respect CXX, CXXFLAGS, LDFLAGS
• Add a soname to the library
• Use INSTALL_LIB to install it