configure: replace AC_PATH_PROG to AC_CHECK_PROG
configure: replace AC_PATH_PROG to AC_CHECK_PROG

Bitcoin Core's `configure` script uses `AC_CHECK_PROG` to find brew in the `PATH` [1]. If found, this macro will set `BREW=brew`. When building with dependencies however the `BREW` variable is set to `no` on macOS via `depends/<host_prefix>/share/config.site` [2] and this overrides `AC_CHECK_PROG` results [3]. Ideally, secp256k1's `configure` script should follow the same logic but this is not what happens because secp256k1's `configure` uses `AC_PATH_PROG` instead which respects preset variable values (in this case for variable `BREW`) only if they are a valid path (i.e., they match `[\\/*] | ?:[\\/]*` [4]), and `no` is not a path.

This commit changes `AC_PATH_PROG` to `AC_CHECK_PROG` to be consistent with Core's `AC_CHECK_PROG`. Both of these macros are supposed to find executables in the `PATH` but the difference is that former is supposed to return the full path whereas the latter is supposed to find only the program. As a result, the latter will accept even non-paths `no` as an override. Not knowing the full path is not an issue for the `configure` script because it will only execute `BREW` immediately afterwards, which works fine without the full path. (In particular, `PATH` cannot have changed in between [5].)

[1] https://github.com/bitcoin/bitcoin/blob/master/configure.ac#L684
[2] https://github.com/bitcoin/bitcoin/blob/master/depends/config.site.in#L73-L76
[3] https://github.com/autotools-mirror/autoconf/blob/6d38e9fa2b39b3c3a8e4d6d7da38c59909d3f39d/lib/autoconf/programs.m4#L47
[4] https://github.com/autotools-mirror/autoconf/blob/6d38e9fa2b39b3c3a8e4d6d7da38c59909d3f39d/lib/autoconf/programs.m4#L127
[5] [3ab1178](https://github.com/bitcoin-core/secp256k1/commit/3ab1178d54029745219d67e6c305df4d7564e278)

bitcoin/secp256k1,bitcoin/secp256k1,bitcoin-core/secp256k1,ElementsProject/secp256k1-zkp,bitcoin-core/secp256k1,ElementsProject/secp256k1-zkp,peterdettman/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,peterdettman/secp256k1