build: use CFL-specific filename macro

[edsiper]

Summary

Follow up on #54 by removing the redundant global __FILENAME__=__FILE__ compiler definition and using a CFL-specific filename macro in the logging interface.

CFL_FILENAME defaults to __FILE__, so compiler path-remapping options such as -ffile-prefix-map work normally. Embedders can override the value without relying on the reserved __FILENAME__ identifier or modifying global CMAKE_C_FLAGS.

Impact

• Removes the remaining global filename compiler definition from CFL's build.
• Replaces the generic reserved identifier with CFL_FILENAME.
• Preserves the existing cfl_report_runtime_error() behavior.
• Keeps the change scoped to CFL; bundled and downstream repositories are unchanged.

Validation

• CFL test suite: 34/34 passed.
• Public-header self-containment tests passed.
• Release build with -ffile-prefix-map passed.
• The resulting CFL archive contains no checkout path, $(subst ...), or $(abspath ...) expression.
• git diff --check passed.