Ensure library link flags are unique, fixing build for Wasm/webR

[georgestagg]

Fixes compiling with Emscripten for webR, where linking is static and so multiple instances of the same library link flag, such as -lz, leads to duplicate symbol errors.

[jeroen]

I think this is not a bug in the package configure but the build tooling. The package simply forwards the flags from pkg-config to the linker. Lots of packages will have this issue. Perhaps we should shim pkg-config to filter duplicates.

[georgestagg]

Perhaps we should shim pkg-config to filter duplicates.

Yes, good idea. As discussed, we should indeed handle this in r-wasm/rwasm. That also means our tweaks will only take effect for Emscripten builds, which is a nice safety net.

The package simply forwards the flags from pkg-config to the linker

I don't think that's strictly true here. Even after shimming pkg-config I'm getting a duplicate symbol error, I think due to the extra -ljpeg added at the end here:

ragg/configure

Lines 37 to 39 in 792c1e1

	echo "Found pkg-config cflags and libs!"
	PKG_CFLAGS=${PKGCONFIG_CFLAGS}
	PKG_LIBS="${PKGCONFIG_LIBS} -ljpeg"

[jeroen]

Ah you are right. I think the problem was actually introduced recently here: fa87a16

Previously we hardcoded libjpeg libs as -ljpeg because it did not used to have a pc file. But then fa87a16 was added, so that results in duplicates and also the configuration failing on systems without a jpeg.pc.

I think fa87a16 was a mistake and should better be reverted, or otherwise we can remove the hardcoded -ljpeg.

[georgestagg]

I'm going to close this in PR favour of r-wasm/rwasm#9, but we should continue the discussion about what to do with -ljpeg. So, I'll open an issue instead and link it here.