[feature] Export cppstd_flag as public tool

[uilianries]

This PR exposes the cppstd_flag method from flags.py to builds:

from conan import ConanFile
from conan.tools.build import cppstd_flag

class FooConanFile(ConanFile):

    def build(self):
        compiler_flag = cppstd_flag(self)
        self.run(f"gcc ... {compiler_flag}")

Changelog: Feature: Make cppstd_flag public to return the corresponding C++ standard flag based on the settings.
Docs: conan-io/docs#3599

closes #12603

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

[uilianries]

This lazy import is to avoid a cyclic import error.

[memsharded]

Thanks!

Will you backport it to Conan 1.X, to achieve recipe compatibility in 1.64?

[uilianries]

Will you backport it to Conan 1.X, to achieve recipe compatibility in 1.64?

That's an excellent idea actually, otherwise, we will not be able to use it in CCI. I'll open a second PR.

[uilianries]

Thanks!

Will you backport it to Conan 1.X, to achieve recipe compatibility in 1.64?

Done, the PR #15731 has the backport implementation.