Fix IFS build inconsistencies

[mikee47]

This PR addresses a couple of issues which have arisen in testing.

Globally define _POSIX_C_SOURCE

Value set by Sming to 200809L making available functionality from the 2008 edition of the POSIX standard (IEEE Standard 1003.1-2008). See https://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html.

We should expect those features to be available generally.
IFS library no longer needs to defined this.

Fix IFS issues

• Fix README links

• Build images using environment variables with regular Windows paths require 'fixing', e.g. "/C/dir/path" -> "C:/dir/path"

• Remove TimeStamp::String() operator completely: it's ambiguous

TimeStamp has implicit time_t cast, which is appropriate.
Calling String(timestamp) could yield the numeric value or the string value. Instead, call toString() method.

For example, when calling Serial.println(timestamp); GCC 10.2 calls TimeStamp::String(), whereas GCC 14.1 uses String(time_t).

[what-the-diff]

PR Summary

• Modification to 'hostlib.h' File
  Remove _POSIX_C_SOURCE definition, no longer required.

• Addition of a Flag in 'build.mk' File
  A new flag has been put into the file -D_POSIX_C_SOURCE=200809L and was added to the CPPFLAGS variable. This change specifies the version of POSIX standard we are using during the building process. Adopting newer standards helps reduce issues related to compatibility and functionality caused by old, outdated guidelines.

• Update to the 'IFS' Subproject Commit
  The commit ID in the 'IFS' file has been altered from 1b4e2f04d4308f0dc966e4d59ee2d6fe5ffd4c2e to af9ddbc666a29b582cd3f1f4ed40a9848a91fe0a. This is an internal identifier change, signifying that we have moved to a newer version of this subproject. This change will likely include bug fixes, feature enhancements, or other improvements made in that newer version.