bug#54378: Still build failed with USE='gtk -X gui'

[Eli Zaretskii]

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> From: "nuil" <nuil@foxmail.com>
> Date: Tue, 15 Mar 2022 14:21:26 +0800
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> Sorry that I may not fully understand you because of my poor English.
> I have tried to build with USE='gtk -X gui', and it still failed
> 
> This is the error I got:
> /usr/bin/x86_64-pc-linux-gnu-ld: pgtkterm.o: undefined reference to symbol 
> 'dlsym@@GLIBC_2.2.5'
> /usr/bin/x86_64-pc-linux-gnu-ld: /lib64/libdl.so.2: error adding symbols: DSO 
> missing from command line
> clang-13: error: linker command failed with exit code 1 (use -v to see 
> invocation)

I guess this means that configuration needs a configure-time test for
linking against libdl, as some systems do need that separate library
to have dlopen/dlsym/etc.