NOTEs in R CMD check

[nevrome]

This is just a quick reminder for @nevrome why these two notes exist

---

> checking for GNU extensions in Makefiles ... NOTE
  GNU make is a SystemRequirements.

This exists because the voro++ Makefile uses some GNU make extensions, which should usually not be used in R packages. Without SystemRequirements: GNU make in the DESCRIPTION file we get

W  checking for GNU extensions in Makefiles
   Found the following file(s) containing GNU extensions:
     src/voro++0.4.6/Makefile
   Portable Makefiles do not use GNU extensions such as +=, :=, $(shell),
   $(wildcard), ifeq ... endif, .NOTPARALLEL See section ‘Writing portable
   packages’ in the ‘Writing R Extensions’ manual.

So this is better.

My attempt to avoid GNU extensions in the Makefile failed. No idea, which operator/function causes the WARNING.

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> checking compilation flags used ... NOTE
  Compilation used the following non-portable flag(s):
    ‘-march=x86-64’

This is a local system setting independent of the package.