Document the need for stdint.h

[hanno-becker]

As pointed out by @gilles-peskine-arm in #884, while stdint.h is a common C90 extension, it is not part of the C90 standard. Our claim of C90 compliance is therefore not true without further qualification.

[mkannwischer]

Agreed, we need to be more precise.
We also need long long - that needs to be explicitly allowed in the CFLAGS through -Wno-long-long.
We bascially followed what OpenSSL requires.