Allow installation of Ruby 2.7, 3.0, 3.1 on Fedora >= 42 (#2543)

Fedora 42+ has updated to GCC v15. GCC v15 changed the default
mode from gnu17 to gnu23: https://gcc.gnu.org/gcc-15/changes.html#c

Ruby < 3.2 can't compile on GCC v15 in the default mode of gnu23.

Fixes #2542

---------

Co-authored-by: Mislav Marohnić <[email protected]>

Author: pboling

bin/ruby-build

@@ -206,6 +206,16 @@ is_freebsd() {
   [ "$(uname -s)" = "FreeBSD" ]
 }
 
+is_fedora() {
+  [ -r /etc/os-release ] || return 1
+  # shellcheck disable=SC1091
+  source /etc/os-release
+  # Treat Aurora, Bazzite, BlueFin, uCore, and other Fedora variants as "fedora".
+  if [[ "${ID_LIKE:-$ID}" != "fedora" || ( $# -gt 0 && "$VERSION_ID" -lt "$1") ]]; then
+    return 1
+  fi
+}
+
 freebsd_package_prefix() {
   local package="$1"
   pkg info --prefix "$package" 2>/dev/null | cut -wf2
@@ -712,6 +722,15 @@ build_package_standard() {
     fi
     if [ -z "$CC" ] && is_mac 1010; then
       export CC=clang
+    elif [ "$ruby_semver" -lt 300200 ] && is_fedora 42; then
+      # Fedora 42+ has updated to GCC v15. GCC v15 changed the default
+      # mode from gnu17 to gnu23: https://gcc.gnu.org/gcc-15/changes.html#c
+      #
+      # Ruby < 3.2 can't compile on GCC v15 in the default mode of gnu23.
+      #
+      # TODO: Consider changing this to check for GCC v15 specifically
+      # rather than inspecting the OS itself.
+      export CFLAGS="-std=gnu17 $CFLAGS"
     fi
     # ./configure --prefix=/path/to/ruby
     # shellcheck disable=SC2086,SC2153