configure should check for compiler and not OS: macOS does support OpenMP, Xcode clang is deficient

[barracuda156]

This code is broken :)

## Check for broken systems produced by a corporation based in Cupertino
{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for macOS" >&5
$as_echo_n "checking for macOS... " >&6; }
RSysinfoName=$("${R_HOME}/bin/Rscript" --vanilla -e 'cat(Sys.info()["sysname"])')
if test x"${RSysinfoName}" == x"Darwin"; then
   { $as_echo "$as_me:${as_lineno-$LINENO}: result: found" >&5
$as_echo "found" >&6; }
   { $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: OpenMP unavailable and turned off." >&5
$as_echo "$as_me: WARNING: OpenMP unavailable and turned off." >&2;}
   openmp_flag="-DARMA_DONT_USE_OPENMP"