Suppress the error caused by Bazel when the headers(hdrs) become absolute path ("/**")

[phoepsilonix]

When pkg-config's cflags only include -I/usr/include for a package, the reference to headers (hdrs) may become an absolute path ("/**"), causing Bazel to generate an error. It seems that _exec_pkg_config does not accept the --keep-system-cflags option.

Description

Currently, when attempting to build fcitx-mozc with Bazel, the output of pkg-config --cflags-only-I is empty. As a result, the hdrs reference becomes an absolute path (/**), causing Bazel to stop due to not finding @fcitx.
If we can avoid this error, the build will be possible. Other packages may not have issues at present, but if cflags only includes -I/usr/include, there is a high likelihood of encountering the same type of error.
This is a patch to suppress that.

Issue IDs

Steps to test new behaviors (if any)

Additional context

[phoepsilonix]

Related fcitx#68

[hiroyuki-komatsu]

The root cause sounds that _exec_pkg_config returns [''] when pkg-config --cflags-only-I returns an empty string. So how about updating _exec_pkg_config as follows instead of modifying _pkg_config_repository_impl?

def _exec_pkg_config(repo_ctx, flag):
    binary = repo_ctx.which("pkg-config")
    if not binary:
        # Using print is not recommended, but this will be a clue to debug build errors in
        # the case of pkg-config is not found.
        print("pkg-config is not found")  # buildifier: disable=print
        return []
    result = repo_ctx.execute([binary, flag] + repo_ctx.attr.packages)
    items = result.stdout.strip().split(" ")

+   # If items is empty, return an empty list.
+   if len(items) == 1 and items[0] == '':
+     return []

    uniq_items = sorted({key: None for key in items}.keys())
    return uniq_items