Make sure nnsvg links against libm in every circumstances

[NiLuJe]

• Link flags on the left of -o: deprecated. May apparently eat your firstborn's soul and/or said link flags ^^.
• Linking order: stuff that requires symbols on the left of stuff that provides those symbols. (And LuaJIT requires libm ;)).

Re koreader/koreader#7190

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This change is 

[Frenzie]

Looks fine, but did you test or should I?

[NiLuJe]

Haven't tested on affected systems (because I don't have any of those ;p), no, so, be my guest ;).

[Frenzie]

Well, I imagine using a pre ~Ubuntu 20.04 to build the whole thing and testing on something newer is part of it, anyway. I'll give it a quick build on my desktop (or not that quick since I haven't built non-debug in quite a while…)

[Frenzie]

Drat, I forgot about this:

cd koreader-appimage-x86_64-linux-gnu && pwd && \
	rm -rf tmp && mkdir -p tmp && \
	cp -Lr koreader tmp && \
	rm -rf tmp/koreader/ota && \
	rm -rf tmp/koreader/resources/icons/src && \
	rm -rf tmp/koreader/spec
/home/frans/src/kobo/koreader/koreader-appimage-x86_64-linux-gnu
cp: cannot stat 'koreader/libs/libbsd.so.0': No such file or directory

Meh, let's just bump it and see what happens. I don't feel like dragging out the ole' Docker image tonight. xP