properly document FPM_CXX, FPM_CXXFLAGS and options

[semarie]

fpm doesn't document C++ compiler / flags neither in README.md neither in fpm help output.

the following PR adds it.

[awvwgk]

Thanks for sharing. Looks good to me.

[semarie]

I am unsure about the reason macos failed. it seems to have failed in help-test (when runned with bootstrap version)

[semarie]

Finally found it:

<INFO>CMD=build/gfortran_2A42023B310FA28D/test//../app/fpm help new        >fpm_scratch_help.txt EXITSTAT=0 CMDSTA
T=0 MESSAGE=

*slurp* empty file fpm_scratch_help.txt
Program received signal SIGSEGV: Segmentation fault - invalid memory reference.
 <ERROR>*swallow* failed to load file fpm_scratch_help.txt

[LKedward]

Apologies @semarie - the CI failure is because macos-latest has changed from macos-11 to macos-12. To get this merged, I've updated your PR to pin the CI runner to macos-11 for now. We will need to fix this CI problem separately.

[LKedward]

Many thanks @semarie! I've now merged this, I'll open a separate issue for the MacOS-12 failure.

[semarie]

thanks for the merge