RPackage is not picking up the correct toolchainopts

[eylenth]

I wanted to install a R package as a stand-alone RPackage.
But I tried to define the following option in the RPackage easyconfig:
toolchainopts = {'cstd': 'c99'}

Then it does not pick up the correct compilation options.

In the beginning of the config-process, it is picking up the correct C99 option:
== 2019-09-17 16:24:18,816 environment.py:97 INFO Environment variable CFLAGS set to -O2 -ftree-vectorize -march=native -fno-math-errno -std=c99 (previously undefined)

but in the actual compilation process, the C99 option is stripped of again:
gcc ... -fpic -O2 -ftree-vectorize -march=native -fno-math-errno -c internal.c -o internal.o

It's using the compiler flags that were used to build R itself with.

cfr. discussion Slack

[boegel]

Workaround that is confirmed to work:

toolchainopts = {'cstd': 'c99'}
preinstallopts = 'export PKG_CFLAGS="$CFLAGS" && '

We need change the RPackage easyblock so PKG_CFLAGS & co are always defined before running R CMD INSTALL to avoid inheriting the compiler flags used to build R itself?