Revert "do not include alloca.h if it does not exist (it is included by default so that we could skip cmake on osx)"

This reverts commit fed987a544ce691f321e1a63c46212e24d09ad60.
no need to #include alloca.h all since on linux alloca is also defined in stdlib.h
no need to include alloca.h as pointed out by @mgumz (merge branch 'kazuho/issues/never-include-alloca.h')