cross-compiling use host g++ to build support/links-dso-program

[William Tambe]

We are porting GLIBC and we are cross-compiling it with GCC without
g++ support; however, cross-compilation is building LINKS_DSO_PROGRAM
using host g++;

In support/Makefile, we are seeing this:

ifeq (,$(CXX))
LINKS_DSO_PROGRAM = links-dso-program-c
else
LINKS_DSO_PROGRAM = links-dso-program
LDLIBS-links-dso-program = -lstdc++ -lgcc -lgcc_s $(libunwind)
endif

Which mean, CXX has been set despite using a cross-compiler that does
not have g++; in other words, CXX get set to host g++.

Any idea how to work-around above issue such that CXX is not set when
using cross-compiler that does not have g++ ?