Don't rely on HAVE_STDINT_H et al. being defined

Not everyone who includes speex_config_types.h will have a test which
defines those, and if we've chosen to use the stdint types at configure
time then we know exactly which header(s) are available, so just choose
the best one then and generate the header to use it.

This fixes ef80120166c3a2552f77008f40c59a84577a36b5 for those users.