grackle linking issues: added FCLIBS to LD_FLAGS

I was having issues linking grackle (latest) when compiling swift with gcc 8.4.0. Adding $FCLIBS to the configure.ac scripts for the grackle test makes the test succeed, and adding it to AM_LD_FLAGS makes the code compile.

According to @lhausammann, another option would be to add FCLIBS inside the variable $GRACKLE_LIBS in configure.ac.