alloca test fails on platforms were it is available

config.test/common/alloca fails on multiple platforms where it is available.

• On mingw32,msvc* there is no alloca.h, but alloca() is defined in malloc.h
• On *BSD, there is no alloca.h, but alloca() is defined in stdlib.h
• On QNX7, there is alloca.h, but it's buggy and needs another header (such as stdlib.h) to define NULL for it.

This causes e.g., qtdeclarative/v4 to use malloc/free where alloca() could be used.