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.

ext/re2/extconf.rb

@@ -94,6 +94,10 @@ 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
@@ -138,10 +142,6 @@ def build_extension
  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; }
@@ -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