Ensure we don't add lre2 to $LIBS

To ensure we link against the correct versions of Abseil and RE2 instead
of any in Ruby's exec_prefix, ensure we don't call `have_library` (which
will add `lre2` to `LIBS`) when compiling the vendored dependencies.

Also: rather than having to set '-x c++' to force try_compile to compile
test programs as C++, use MakeMakefile's own native support for C++.

ext/re2/extconf.rb

@@ -11,6 +11,8 @@
 require 'mkmf'
 require_relative 'recipes'
 
+include MakeMakefile['C++']
+
 module RE2
   class Extconf
     def configure
@@ -94,6 +96,10 @@ def build_with_system_libraries
       ]
 
       dir_config("re2", header_dirs, lib_dirs)
+
+      unless have_library("re2")
+        abort "You must have re2 installed and specified with --with-re2-dir, please see https://github.com/google/re2/wiki/Install"
+      end
     end
 
     def build_with_vendored_libraries
@@ -130,23 +136,17 @@ def build_extension
       $CFLAGS << " -Wall -Wextra -funroll-loops"
       $CPPFLAGS << " -Wno-register"
 
-      # Pass -x c++ to force gcc to compile the test program
-      # as C++ (as it will end in .c by default).
-      compile_options = +"-x c++"
-
       have_library("stdc++")
       have_header("stdint.h")
       have_func("rb_gc_mark_movable") # introduced in Ruby 2.7
 
-      unless have_library("re2")
-        abort "You must have re2 installed and specified with --with-re2-dir, please see https://github.com/google/re2/wiki/Install"
-      end
-
       minimal_program = <<~SRC
         #include <re2/re2.h>
         int main() { return 0; }
       SRC
 
+      compile_options = +''
+
       re2_requires_version_flag = checking_for("re2 that requires explicit C++ version flag") do
         !try_compile(minimal_program, compile_options)
       end
@@ -232,20 +232,22 @@ def static_pkg_config(pc_file, pkg_config_paths)
 
       # Replace all -l flags that can be found in one of the static library
       # paths with the absolute path instead.
-      libflags = minimal_pkg_config(pc_file, '--libs-only-l', '--static')
+      minimal_pkg_config(pc_file, '--libs-only-l', '--static')
         .shellsplit
-        .map do |flag|
-          next flag unless flag.start_with?('-l')
-
-          lib = "lib#{flag.delete_prefix('-l')}.#{$LIBEXT}"
-          static_lib_path = static_library_paths.find { |path| File.exist?(File.join(path, lib)) }
-          next flag unless static_lib_path
-
-          File.join(static_lib_path, lib)
+        .each do |flag|
+          if flag.start_with?('-l')
+            lib = "lib#{flag.delete_prefix('-l')}.#{$LIBEXT}"
+
+            if (static_lib_path = static_library_paths.find { |path| File.exist?(File.join(path, lib)) })
+              $LDFLAGS << ' ' << File.join(static_lib_path, lib)
+            else
+              append_ldflags(flag.shellescape)
+            end
+          else
+            append_ldflags(flag.shellescape)
+          end
         end
 
-      $libs = [libflags, $libs].join(" ").strip
-
       incflags = minimal_pkg_config(pc_file, '--cflags-only-I')
       $INCFLAGS = [incflags, $INCFLAGS].join(" ").strip