Rodrigo Mesquita authored May 19, 2023 and Marge Bot committed Jun 28, 2023

There is a distinction to be made between the Haskell Preprocessor and
the C preprocessor. The former is used to preprocess Haskell files,
while the latter is used in C preprocessing such as Cmm files.

In practice, they are both the same program (usually the C compiler) but
invoked with different flags.

Previously we would, at configure time, configure the haskell
preprocessor and save the configuration in the settings file, but,
instead of doing the same for CPP, we had hardcoded in GHC that the CPP
program was either `cc -E` or `cpp`.

This commit fixes that asymmetry by also configuring CPP at configure
time, and tries to make more explicit the difference between HsCpp and
Cpp (see Note [Preprocessing invocations]).

Note that we don't use the standard CPP and CPPFLAGS to configure Cpp,
but instead use the non-standard --with-cpp and --with-cpp-flags.
The reason is that autoconf sets CPP to "$CC -E", whereas we expect the
CPP command to be configured as a standalone executable rather than a
command. These are symmetrical with --with-hs-cpp and
--with-hs-cpp-flags.

Cleanup: Hadrian no longer needs to pass the CPP configuration for CPP
         to be C99 compatible through -optP, since we now configure that
         into settings.

Closes #23422