- inline Pervasives in Stdlib and re-add Pervasives as a deprecated
module that aliases all elements of Stdlib except the stdlib modules.

- remove special case for Stdlib.Pervasives in printtyp.ml

It has been deprecated since 2000, shown a deprecation warning
since 4.02, and Sort.merge is documented to have undefined behavior
when the lists being merged are not sorted in the first place.

Makefiles: consistently offer `allopt` and `opt.opt` targets

This commit renames a few C compiler related build variables so that
they are reserved for the build system. They will then be re-introduced,
but this time as user varialbes whose value can be freely customized
when compiling the package, without risking to conflict with those
command-line flags that are required by the build system itself.

Here are the variables this commit renames:

- CFLAGS -> OC_CFLAGS
- CPPFLAGS -> OC_CPPFLAGS
- LDFLAGS -> OC_LDFLAGS

Note: before this commit the compilation of scheduler.c in
otherlibs/threads was relying on make's implicit rule to compile C files.

Since this commit stops using the standard variables for flags,
it is necessary to introduce an explicit rule to compile C files
and that makes use of the newly introduced variables.

Semantically unchanged, just moved some expressions into separate
definitions.