2064: Use system libblake if available

[xw19]

Use system libblake if available.

Fixes: #2064

[gemini-code-assist]

Code Review

This pull request introduces a configuration option to toggle between using an embedded version of the blake3 library and a system-provided one. The build system and test scripts have been updated to support this conditional compilation. Feedback was provided regarding the detection logic for the system libblake3 in configure.ac, specifically noting that the current implementation may fail in non-standard environments because it doesn't correctly propagate flags from pkg-config and uses a misleading error message.

[gemini-code-assist]

The detection logic for system libblake3 has a few issues:

1. The error message in AC_SEARCH_LIBS is misleading; it mentions 'headers not found' while checking for the library.
2. AC_SEARCH_LIBS is called before PKG_CHECK_MODULES. If the library is in a non-standard path, AC_SEARCH_LIBS will fail before pkg-config can provide the correct paths.
3. The flags from PKG_CHECK_MODULES (BLAKE3_CFLAGS and BLAKE3_LIBS) are not propagated to the build system, which may cause compilation or linking failures in environments with non-standard installation paths.

Consider using PKG_CHECK_MODULES as the primary check and falling back to AC_SEARCH_LIBS only if the .pc file is missing.

AM_COND_IF([HAVE_EMBEDDED_BLAKE3], [], [
  PKG_CHECK_MODULES([BLAKE3], [libblake3], [
    AC_DEFINE([HAVE_BLAKE3], 1, [Define if libblake3 is available])
    CFLAGS="$CFLAGS $BLAKE3_CFLAGS"
    LIBS="$LIBS $BLAKE3_LIBS"
  ], [
    AC_SEARCH_LIBS(blake3_hasher_init, [blake3], [AC_DEFINE([HAVE_BLAKE3], 1, [Define if libblake3 is available])], [AC_MSG_ERROR([*** libblake3 not found])])
  ])
])

[packit-as-a-service]

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