Improve libxml2 detection

[krlmlr]

#1197 (comment)

• Rename xml2_include_paths variable to xml2_cflags
• Also use linker flags by xml2-config or pkg-config instead of hard-coding them

[szhorvat]

I checked both boxes here, as they're done. But there's one more thing: if libxml2 is not found, it'd be nice to just disable GraphML support instead of failing to build.

[maelle]

just disable GraphML support

What should then happen if the user tries using the related functionality?

I wonder whether other R packages also have this sort of "partial installation".

[szhorvat]

There will be a nice and clear error message explaining that this functionality is not available. This is all already implemented, so nothing needs to be done in R code. The C sources simply need to be compiled with the appropriate macros (un)defined.

[maelle]

@Antonov548 can you please have a look?

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