Fix test_ecbuild_override_compiler_flags (fixes #135)

[wdeconinck]

Description

This PR fixes the test_ecbuild_override_compiler_flags reported failing in #135 .
The problem was that the file 'compiler-flags.cmake' needed the "${CMAKE_CURRENT_SOURCE_DIR}/" prefixed.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[marcosbento]

This looks OK!