Put -std=gnu17 on CC for Ruby 2.7 too, its configure ignores cflags

bin/ruby-build

@@ -755,8 +755,9 @@ build_package_standard() {
       # Ruby < 3.2 can't compile as C23, where `()` in a declaration means `(void)`.
       extra_cflags="-std=gnu17"
     fi
-    if [[ -n "$extra_cflags" && "$ruby_semver" -ge 20600 && "$ruby_semver" -lt 20700 ]]; then
-      # Ruby 2.6 checks its MJIT header with a bare $(CC), so the flags must be part of CC there.
+    if [[ -n "$extra_cflags" && "$ruby_semver" -ge 20600 && "$ruby_semver" -lt 20800 ]]; then
+      # Ruby 2.6 checks its MJIT header with a bare $(CC), and the configure script of Ruby 2.7
+      # (generated by Autoconf 2.71) silently ignores cflags, so the flags go on CC for those.
       cc_override="$cc $extra_cflags"
       extra_cflags=
     fi

test/compiler.bats

@@ -190,6 +190,27 @@ OUT
   unstub_linux_build
 }
 
+@test "GCC 15 compiles Ruby 2.7 as gnu17 via CC" {
+  stub_linux_build
+  stub_cc '"#define __GNUC__ 15"'
+
+  run_inline_definition <<DEF
+build_package_standard ruby-2.7.8
+DEF
+  assert_success
+  run cat build.log
+  assert_output <<OUT
+./configure --prefix=$INSTALL_ROOT --with-ext=openssl,psych,+
+CC=gcc -std=gnu17
+cflags=unset
+make -j 2
+make install
+OUT
+
+  unstub gcc
+  unstub_linux_build
+}
+
 @test "GCC 15 compiles Ruby <= 2.2 as gnu17" {
   stub_linux_build
   stub_cc '"#define __GNUC__ 15"'