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](3ab1178)

configure.ac

@@ -42,8 +42,8 @@ AM_PROG_AS
 case $host_os in
   *darwin*)
      if  test x$cross_compiling != xyes; then
-       AC_PATH_PROG([BREW],brew,)
-       if test x$BREW != x; then
+       AC_CHECK_PROG([BREW], brew, brew)
+       if test x$BREW = xbrew; then
          # These Homebrew packages may be keg-only, meaning that they won't be found
          # in expected paths because they may conflict with system files. Ask
          # Homebrew where each one is located, then adjust paths accordingly.
@@ -58,10 +58,10 @@ case $host_os in
            VALGRIND_CPPFLAGS="-I$valgrind_prefix/include"
          fi
        else
-         AC_PATH_PROG([PORT],port,)
+         AC_CHECK_PROG([PORT], port, port)
          # If homebrew isn't installed and macports is, add the macports default paths
          # as a last resort.
-         if test x$PORT != x; then
+         if test x$PORT = xport; then
            CPPFLAGS="$CPPFLAGS -isystem /opt/local/include"
            LDFLAGS="$LDFLAGS -L/opt/local/lib"
          fi