GCC 10 and Makefile fixes

[SoapGentoo]

Hi @lh3,
This PR includes a patch for the upcoming GCC 10, which imports C++'s ODR rule to C too.
I've also fixed the Makefile a bit so we don't have to patch it downstream. CC and AR don't need to be defined, make will use cc and gcc in their place. By omitting the definition, we don't have to hack the Makefile because we export CC and AR to the environment, which Make looks for before using the fallbacks.

[jmarshall]

The Makefile changes are already PR #263. (Further Makefile breakage to use environment variables first is unnecessary — even though bwa has no configure script — as your recipe would use make -e or make CC="$CC" AR="$AR" etc. Cf samtools/samtools#533.)

[SoapGentoo]

most build environments export CC without calling make CC="${CC}" explicitly, which just means you need to override the whole default build rule again.

The changes in PR #263 are good, but I'd like to see CC and AR removed, because there is absolutely no need to define them, and removing them breaks nothing.

[SoapGentoo]

@jmarshall also, thanks for your kind words on Twitter, pretty rich coming from someone who can't write a build system that builds out of source, or write portable Makefiles, or just plain Autoconf, which you seem to be advocating, yet can't seem to get right yourself (:wave: samtools/htslib@3472494#diff-67e997bcfdac55191033d57a16d1408a).

[jmarshall]

That htslib commit fixed a minor bug introduced by someone else.

I suggest you keep any discussion here on topic for bwa.

[lh3]

@jmarshall & @SoapGentoo I wonder if Makefile in minimap2 solves the problem. If so, I may change bwa makefile to the minimap2 way.

[SoapGentoo]

@lh3 that Makefile is fine as well, as it doesn't explicitly set the CC variable 👍

[SoapGentoo]

@lh3 I've dropped the Makefile changes, could you merge the GCC 10 fix?

[lh3]

Thanks.