Non-portable flags in Makevars

[jack89roberts]

rcmdcheck complains about Makevars having non-portable flags/extensions:

❯ checking compilation flags in Makevars ... WARNING
  Non-portable flags in variable 'PKG_CPPFLAGS':
    -fno-fast-math -msse2 -mfpmath=sse -mstackrealign

❯ checking for GNU extensions in Makefiles ... WARNING
  Found the following file(s) containing GNU extensions:
    src/Makevars
  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.

https://github.com/alan-turing-institute/network-comparison/runs/6691885148?check_suite_focus=true

Kind of related to #137 (@andeElliott @ots22 )

[jack89roberts]

@ots22 @andeElliott - do we actually need to append to PKG_CPPFLAGS in Makevars or is setting it as in 9f7453b fine?

[jack89roberts]

though that only fixes the 2nd warning above, not the 1st