Require C++17 on macOS

To be able to install the package on older R versions,
e.g. R 3.6 to R 4.2, which default to C++14 or C++11.
This is surely ok for macOS systems since Big Sur.

tools/config/configure.R

@@ -182,7 +182,7 @@ for (src_ in gle_cc) {
 pkg_sources <- sort(dir("./src", ".cpp$|.c$"), decreasing = TRUE)
 
 # compiler flags
-cxxflags <- ""
+cxxflags <- if (mac) "-std=gnu++17" else ""
 
 fix_flags <- function(x) {
   x <- gsub("-Wno-float-conversion ", "", x, fixed = TRUE)