This is tricky, because autoconf provides no way to do this automatically
(AC_PROG_CXX fails if no C++ compiler).
One could override AC_MSG_ERROR/FAILURE for AC_PROG_CXX, but that causes all
sorts of problems later (like sys/types.h not found).
So instead create yet another configure in libclamav/c++,
but teach autoconf not to fail if the subconfigure fails
(by overriding AC_MSG_ERROR to set a variable instead of exiting).
This seems to work with shared and static builds, "-all-static" builds
don't work yet, will fix that later.