mk: add support for passing LDFLAGS to libs and bins

autotools-based systems usually allow user to
append own LDFLAGS like
    LDFLAGS="-Wl,-O1 -Wl,--as-needed -Wl,--hash-style=gnu"
at ./configure stage

This change plumbs LDFLAGS through similar to existing CXXFLAGS variable.

Signed-off-by: Sergei Trofimovich <siarheit@google.com>