makefile: increase portability

Fix bogus use 'g++'.  On some systems 'g++' is more correctly spelled
'g++48' or the like.
makefile: fix build

boost no longer uses the 'mt' suffix.
makefile: remove chatty messages

make(1) is already chatty for you.  You can even make it more verbose.
Adding manual echo lines is pointless.
makefile: Fix toolchain invocations

Most users expect CXXFLAGS and LDFLAGS to be used.
makefile: don't rebuild libzerocoin.so if it already exists
makefile: don't fail make clean if no objects exist.

rm -f is safe here :))
bitcoin_bignum/serialize.h: duplicate declaration of insert function.

Without this the C++ code could not compile:

bitcoin_bignum/serialize.h:911:10: error: class member cannot be redeclared
    void insert(iterator it, std::vector<char>::const_iterator first, std::vector<char>::const_iterator last)
         ^
bitcoin_bignum/serialize.h:898:10: note: previous declaration is here
    void insert(iterator it, const_iterator first, const_iterator last)
         ^
makefile: use c++11

The C++ code uses 'long long' which is a C++11 feature, so depend on it.