lib: posix: Switch to use zephyr_interface_library_named cmake directive

Similar to how other sub-libraries are defined in Zephyr tree, e.g.
"fs", "lgvl", etc. This is supposed to help with the need to
explicitly add posix include path to each and every application using
POSIX subsys.

Fixes: #15627

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>

lib/posix/CMakeLists.txt

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 
-add_library(posix_subsys INTERFACE)
+zephyr_interface_library_named(posix_subsys)
 
 target_include_directories(posix_subsys INTERFACE ${ZEPHYR_BASE}/include/posix)
 

lib/posix/Kconfig

@@ -96,4 +96,12 @@ config	POSIX_MAX_OPEN_FILES
 endif
 endif # FILE_SYSTEM
 
+# The name of this option is mandated by zephyr_interface_library_named
+# cmake directive.
+config APP_LINK_WITH_POSIX_SUBSYS
+        bool "Make POSIX headers available to application"
+        default y
+        help
+          Add POSIX subsystem header files to the 'app' include path.
+
 endif # POSIX_API