target_link_libraries: Fix regression in case of $<CONFIG> genex
Since commit b8626261e9 (Precompile headers: Add methods to generate PCH
sources, 2019-07-13, v3.16.0-rc1~182^2~4) we look up source files for a
target using an upper-case configuration even though an original-case
name is sufficient. Since commit 36ded610af (PCH: Generate sources
during Compute step, 2019-10-05, v3.16.0-rc1~2^2) the source file lookup
is the first time we compute many on-demand structures that depend on
the configuration name. This caused the `$<CONFIG>` generator
expression to evaluate to the upper-case configuration name in some
cases where we used original-case before.
Fix this by switching the source file lookup to the original-case config
name. Add a test covering the symptom that led to the discovery of this
problem.
Fixes: #20517
5 files changed
