optimize CMakeBuildDirChooser::buildDirSettings()

Summary:
-if we have a match for a key, no other key can match
-match the start of a new line as QByteArray instead of QString, most lines will
not match and all keys are plain ASCII, this saves a lot of needless
conversions and memory allocations
-modify the original line instead of doing a copy, the original one will not
be used again anyway.

Test Plan: Open and build a CMake-based project.

Reviewers: mwolff

Reviewed By: mwolff

Subscribers: mwolff, apol, kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D18857