CXXFLAGS_CXX11 is for the compiler used to compile LO proper.  The plugin needs
to be compiled in a way compatible with compiling Clang, and the compiler and
any relevant flags can be controlled with COMPILER_PLUGINS_CXX.  (And at least
on macOS when compiling LO against a locally-built recent Clang trunk,
CXXFLAGS_CXX11 will now contain -std=gnu++17, but COMPILER_PLUGINS_CXX can still
point at Apple's Xcode clang++, which does not understand -std=gnu++17.)

Also, if COMPILER_PLUGINS_CXX is not set, simply default it to g++ instead of
trying to construct an acceptable CLANGCXX value from CXX (which would be
Clang).  (The problem with using Clang without CXXFLAGS_CXX11 is that Clang,
unlike GCC, typically defaults to C++03, but building compilerplugins requires
C++11 at least.  That would cause e.g. the Gerrit/Jenkins linux_clang_dbgutil_64
builds to fail---but which also needs COMPILER_PLUGINS_CXX to be explicitly set
to "g++ -std=c++11" as GCC on those machines is still 4.8.5 defaulting to
C++03.)

Change-Id: Id4ee4e54fa871cb6e621069cd050ae5b31922b34
Reviewed-on: https://gerrit.libreoffice.org/45856Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>