cli: Warn if anchor-spl/idl-build is missing

[acheroncrypto]

Problem

Not enabling anchor-spl's idl-build feature can result in cryptic compilation errors during IDL build process:

error[E0599]: no function or associated item named `create_type` found for struct `anchor_spl::token::TokenAccount` in the current scope
   --> programs/debug/src/lib.rs:263:10
    |
263 | #[derive(Accounts)]
    |          ^^^^^^^^ function or associated item not found in `TokenAccount`
    |
    = note: this error originates in the derive macro `Accounts` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no associated item named `DISCRIMINATOR` found for struct `anchor_spl::token::TokenAccount` in the current scope
   --> programs/debug/src/lib.rs:263:10
    |
263 | #[derive(Accounts)]
    |          ^^^^^^^^ associated item not found in `TokenAccount`
    |
    = note: this error originates in the derive macro `Accounts` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no function or associated item named `insert_types` found for struct `anchor_spl::token::TokenAccount` in the current scope
   --> programs/debug/src/lib.rs:263:10
    |
263 | #[derive(Accounts)]
    |          ^^^^^^^^ function or associated item not found in `TokenAccount`
    |
    = note: this error originates in the derive macro `Accounts` (in Nightly builds, run with -Z macro-backtrace for more info)

Summary of changes

Log a warning if anchor-spl/idl-build is missing:

WARNING: `idl-build` feature of `anchor-spl` is not enabled. This is likely to result in cryptic compile errors.

        To solve, add `anchor-spl/idl-build` to the `idl-build` feature list:

        [features]
        idl-build = ["anchor-spl/idl-build", ...]

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