Clarify of implicitly-included headers in C-like compiler checks

Compiler check methods compiler.compute_int(), compiler.alignment() and compiler.sizeof() now have their implicitly-included headers corrected and documented.

<stdio.h> was included unintentionally when cross-compiling, which is less than ideal because there is no guarantee that a standard library is available for the target platform. Only <stddef.h> is included instead.

For projects that depend on the old behavior, the compiler check methods have an optional argument prefix, which can be used to specify additional #include directives.