[ci] use lowercase library names in linker flags

[characat0]

This change should improve compatibility and consistency with other projects and build tools, and prevent any potential issues arising from the inconsistent capitalization of the library names.

[jameslamb]

Thanks for taking the time to contribute to LightGBM!

Can you give some specific examples of "potential issues arising from the inconsistent capitalization of the library names" to help us understand this proposed change?

[characat0]

I was trying to create libraries for Julia using BinaryBuilder.jl, during compilation I found the following errors for windows:

/opt/x86_64-w64-mingw32/bin/../lib/gcc/x86_64-w64-mingw32/7.1.0/../../../../x86_64-w64-mingw32/bin/ld: cannot find -lWs2_32
/opt/x86_64-w64-mingw32/bin/../lib/gcc/x86_64-w64-mingw32/7.1.0/../../../../x86_64-w64-mingw32/bin/ld: cannot find -lIPHLPAPI

when refering to the documentation of the builders, I found that the libraries were in lowercase for linux, and since windows is case insensitive, I think this change would allow more projects to build LightGBM without making downstream patches.

[jameslamb]

Works for me! We don't use MinGW to cross-compile Windows artifacts on Linux (we just directly compile on Windows), so haven't run into the issues you've described.

But we've been using all-lowercase library names in the same linker flags for CMake-driven builds of the R package on Windows for a few years now, without issue:

LightGBM/R-package/src/Makevars.win.in

Lines 18 to 22 in 7c71db0

	PKG_LIBS = \
	${SHLIB_OPENMP_CXXFLAGS} \
	${SHLIB_PTHREAD_FLAGS} \
	-lws2_32 \
	-lIphlpapi

So given that + all the passing CI jobs, I'm confident this change won't cause issues.

Thanks for the contribution!

[jameslamb]

Oh wait actually I just realized there's a capital I there. Could you please update that place too?

LightGBM/R-package/src/Makevars.win.in

Line 22 in 7c71db0

-lIphlpapi

[characat0]

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[jameslamb]

looks great, thanks so much!

[characat0]

It's great to start contributing to the libraries I use
Thank you for your guidance

[jameslamb]

of course! We'd love to have more of your help here in LightGBM if you have time and interest 😁

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