Build failure without libpng installed with -DGDAL_USE_PNG=OFF

[thesamesam]

Expected behavior and actual behavior.

When libpng is not installed, the build fails:

-- The following RECOMMENDED packages have not been found:

 * PNG
   PNG compression library (external)
 * QHULL
   Enable QHULL (external)
 * LERC
   Enable LERC (external)
 * BRUNSLI
   Enable BRUNSLI for JPEG packing in MRF

-- Disabled components:

 * MYSQL component has been detected, but is disabled with GDAL_USE_MYSQL=no
 * ZSTD component has been detected, but is disabled with GDAL_USE_ZSTD=no
 * JPEG component has been detected, but is disabled with GDAL_USE_JPEG=OFF, and the internal library is also disabled with GDAL_USE_JPEG_INTERNAL=OFF
 * SQLITE3 component has been detected, but is disabled with GDAL_USE_SQLITE3=no
 * WEBP component has been detected, but is disabled with GDAL_USE_WEBP=no
 * POSTGRESQL component has been detected, but is disabled with GDAL_USE_POSTGRESQL=no
 * LIBLZMA component has been detected, but is disabled with GDAL_USE_LIBLZMA=no
 * LZ4 component has been detected, but is disabled with GDAL_USE_LZ4=OFF
 * OPENJPEG component has been detected, but is disabled with GDAL_USE_OPENJPEG=no


-- Internal libraries enabled:

 * SHAPELIB internal library enabled


-- <<< Gentoo configuration >>>
Build type      RelWithDebInfo
Install path    /usr
Compiler flags:
C               -O2 -pipe -march=native -fdiagnostics-color=always -frecord-gcc-switches     -ggdb3 -Werror=format-security -Werror=implicit-function-declaration -fvisibility=hidden
C++             -O2 -pipe -march=native -fdiagnostics-color=always -frecord-gcc-switches -D_GLIBCXX_ASSERTIONS -ggdb3 -Werror=format-security -fvisibility=hidden
Linker flags:
Executable      -Wl,-O1 -Wl,--as-needed -Wl,--defsym=__gentoo_check_ldflags__=0
Module          -Wl,-O1 -Wl,--as-needed -Wl,--defsym=__gentoo_check_ldflags__=0 -Wl,--no-undefined
Shared          -Wl,-O1 -Wl,--as-needed -Wl,--defsym=__gentoo_check_ldflags__=0 -Wl,--no-undefined

-- Configuring done
CMake Error at gdal.cmake:478 (target_link_libraries):
  Target "GDAL" links to:

    PNG::PNG

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

Call Stack (most recent call first):
  CMakeLists.txt:218 (include)


-- Generating done
CMake Generate step failed.  Build files cannot be regenerated correctly.
 * ERROR: sci-libs/gdal-3.5.0-r2::gentoo failed (configure phase):
 *   cmake failed

Full log: build.log

Steps to reproduce the problem.

1. Have a system without libpng installed.
2. Attempt to build gdal using the CMake build system with -DGDAL_USE_PNG=OFF

Operating system

Gentoo

GDAL version and provenance

3.5.0

[thesamesam]

Very similar to #5741. I don't know enough about GDAL to quickly spot where the missing conditionals are, as I odn't know which plugins make sense without PNG (and JPEG -- which is harder to follow as an outsider b/c of the internal version too) respectively.

[thesamesam]

Thanks a lot for the fixes!

[DrChai]

Just built it without exception. Thanks for the quick fix!