[Development] ERROR: Feature 'webengine-system-libwebp' was enabled, but the pre-condition 'libs.webengine-webp' failed.

[YuGiOhJCJ Mailing-List]

Hello,

I am trying to build Qt 5.10.1 on Slackware64 14.2 with the "-system-webengine-webp" option and libwebp 0.6.1:
---
$ ./configure -v \
[...]
	-system-assimp \
	-system-doubleconversion \
	-system-freetype \
	-system-harfbuzz \
	-system-libjpeg \
	-system-libpng \
	-system-pcre \
	-system-sqlite \
	-system-xcb \
	-system-webengine-icu \
	-system-webengine-ffmpeg \
	-system-webengine-opus \
	-system-webengine-webp \
	-system-zlib \
[...]
	-pulseaudio
[...]
Note: Also available for Linux: linux-clang linux-icc

Note: -headerdir is not a subdirectory of -prefix.
Note: -libdir is not a subdirectory of -prefix.
Note: -docdir is not a subdirectory of -prefix.

Note: -optimized-tools is not useful in -release mode.

Note: Dropped compiler flags '-pthread' when detecting library 'glib'.

Note: Dropped compiler flags '-pthread' when detecting library 'gtk3'.

Note: No wayland-egl support detected. Cross-toolkit compatibility disabled.

Note: Dropped compiler flags '-pthread' when detecting library 'gstreamer'.

Note: Dropped compiler flags '-pthread' when detecting library 'gstreamer_app'.

Note: Dropped compiler flags '-pthread' when detecting library 'webengine-protobuf'.

ERROR: Feature 'webengine-system-libwebp' was enabled, but the pre-condition 'libs.webengine-webp' failed.

ERROR: Feature 'webengine-system-ffmpeg' was enabled, but the pre-condition 'libs.webengine-ffmpeg && features.webengine-system-opus && features.webengine-system-libwebp' failed.
---

As you can see in the output above, the error is about the libwebp library that is not found (or at least that is not working correctly).

However, I found the test file that is checking the libwebp library and I tried it manually:
---
$ cd qtimageformats/config.tests/libwebp
$ qmake libwebp.pro
$ make
g++ -c -pipe -O2 -Wall -W  -I/usr/lib64/qt/mkspecs/linux-g++ -I. -o libwebp.o libwebp.cpp
libwebp.cpp: In function ‘int main(int, char**)’:
libwebp.cpp:40:20: warning: unused variable ‘output_buffer’ [-Wunused-variable]
     WebPDecBuffer *output_buffer = &config.output;
                    ^~~~~~~~~~~~~
libwebp.cpp:41:28: warning: unused variable ‘bitstream’ [-Wunused-variable]
     WebPBitstreamFeatures *bitstream = &config.input;
                            ^~~~~~~~~
libwebp.cpp:42:17: warning: variable ‘picture’ set but not used [-Wunused-but-set-variable]
     WebPPicture picture;
                 ^~~~~~~
libwebp.cpp:44:16: warning: variable ‘config2’ set but not used [-Wunused-but-set-variable]
     WebPConfig config2;
                ^~~~~~~
libwebp.cpp:47:18: warning: unused variable ‘demuxer’ [-Wunused-variable]
     WebPDemuxer *demuxer = WebPDemux(&data);
                  ^~~~~~~
libwebp.cpp:48:18: warning: variable ‘iter’ set but not used [-Wunused-but-set-variable]
     WebPIterator iter;
                  ^~~~
---

So, the test file is working.
I am able to build it manually.

Why the Qt build system is complaining about this library installed on my system please?

Thank you.
Best regards.