A funny thing happened while refactoring the function argument parsing
in the `PNGGenConfig` module: the processing of dependencies inside
the CMake function `generate_copy` got fixed, thanks to an inadvertent
typo correction!
Going down the rabbit hole, we uncovered two issues:
1. A typo in `scripts/cmake/PNGGenConfig.cmake` caused a dependency
declaration to disappear, disrupting the graph. Fortunately, this
disruption was being mitigated by an additional set of (redundant)
symbolic target declarations that kept the CMake build going.
2. The exact string matching inside `scripts/cmake/gensrc.cmake.in`
imposed an artificial limitation, disallowing the use of absolute
file paths.
The typo correction in (1), and the use of regex matching instead of
exact string matching in (2), allowed us to use absolute file paths
in the main CMake file, consistently, thus allowing CMake to recognize
them as uniquely-identifiable nodes in the dependency graph. There
should be no further need to have extra symbolic targets for proper
node identification.
In the end, we shaved off the redundant symbolic target declarations,
retaining only `png_genfiles` and `png_genprebuilt`.
In preparation for the upcoming changes in the build system, any parts
of the CMake file that are unlikely to be affected should be moved out
of the way. This should facilitate an easier resync between the branch
'libpng16' and its successor(s).
Specifically:
* Move the functions `generate_chk`, `generate_out`, `generate_source`
and `generate_copy` to scripts/cmake/PNGGenConfig.cmake.
* Move the function `png_add_test` to scripts/cmake/PNGTest.cmake.
* Leave the function `create_symlink` in place, but add a TODO note.
As we raised the minimum required CMake version to 3.14, we should
now be able to use CMake's built-in function instead.
