build: add _Generic selection test to autoconf

[underwoo]

configure will fail with a note if the C compiler does not support the _Generic selection introduced in C11 - https://en.cppreference.com/w/c/language/generic

Fixes #102

[ngs333]

@underwoo
Was this tested?

[ngs333]

Will wait for the merge of upstream PR before testing and merging.

[ceblanton]

I tried this and it did not work as expected. As a control case, I commented out the module load gcc in the GFDL-WS site file, then did a regular compilation. ./configure worked and the compile failed with the telltale syntax errors:

create_xgrid.c(1311): error: expected an expression
          if(fabs(determ) < EPSLN30) {

Then I reran with these changes, starting with a fresh clean and autoreconf -i. ./configure now barks with:

./configure: line 4675: GX_C__GENERIC: command not found
./configure: line 4676: test: =: unary operator expected

but still passes and create the Makefiles.

[ngs333]

@ceblanton , @underwoo
I ran into the exact smame issue Chris shows above (#146 (review)). I was using analysis.

[underwoo]

I just noticed I forgot to add the gx_c__generic.m4 file to the commit. I'll get that added, and push.

[underwoo]

@ngs333 @ceblanton I pushed the GC_C__GENERIC m4 macro. Please try again.

[ngs333]

I verified the failure and appropriate message for the case when the compiler is not compliant.