prefer if expressions

[pjljvandelaar]

Dear Gprbuild developers,

Since Ada2012 the if expression is supported. Using the if expression in many cases improves the readability of the code.

Greetings, Pierre

P.S. When you see no if expression in the change, the code is simplified even further to improve readability.

Problem detected and solved by Rejuvenation-Ada crate

[pjljvandelaar]

After this pull request, you might want to change some functions into expression functions to make the code even more readable!