Add mergeback for -D and -W cflags to match pkgconf behavior (#43)

When multiple packages have the same -D or -W flags, pkgconf removes
previous occurrences and keeps only the last one (merge back).
This prevents flag duplication like "-DNOMINMAX"
appearing many times.

```console
$ pkgconf --cflags re2
-pthread -DNOMINMAX
$ ruby -r pkg-config -e 'puts PKGConfig.cflags("re2")'
-pthread -DNOMINMAX -DNOMINMAX -DNOMINMAX ...
```

Flags excluded from merge back (kept as duplicates):
- -Wa, (assembler options)
- -Wl, (linker options)
- -Wp, (preprocessor options)

This PR only supports merge back for -D and -W flags. Other flags that
pkgconf also merges back are not yet supported.

Author: otegami

lib/pkg-config.rb

@@ -545,6 +545,7 @@ def collect_cflags
         flag.gsub(/\A-I/, "/I")
       end
     end
+    other_flags = merge_back_cflags(other_flags)
     [path_flags, other_flags]
   end
 
@@ -579,6 +580,32 @@ def normalize_cflags(cflags)
     normalized_cflags
   end
 
+  # Implementing behavior compatible with pkgconf's pkgconf_fragment_copy().
+  # This is not a complete reproduction yet, but the goal is to stay compatible.
+  # https://github.com/pkgconf/pkgconf/blob/pkgconf-2.5.1/libpkgconf/fragment.c#L381-L416
+  def merge_back_cflags(cflags)
+    merge_backed_cflags = []
+    cflags.each do |cflag|
+      if mergeable_flag?(cflag)
+        # NOTE: This may be slow because this checks merge_back_cflags N times
+        # (where N is the number of mergeable flags).
+        merge_backed_cflags.delete(cflag)
+      end
+      merge_backed_cflags << cflag
+    end
+    merge_backed_cflags
+  end
+
+  def mergeable_flag?(flag)
+    return false unless flag.start_with?("-")
+    return true if flag.start_with?("-D")
+    if flag.start_with?("-W")
+      return false if flag.start_with?("-Wa,", "-Wl,", "-Wp,")
+      return true
+    end
+    false
+  end
+
   def collect_libs
     target_packages = [*required_packages, self]
     libs_set = []

test/test-pkg-config.rb

@@ -314,4 +314,34 @@ def test_equals_to
                    parse_requires("fribidi = 1.0"))
     end
   end
+
+  sub_test_case("#merge_back_cflags") do
+    def merge_back_cflags(cflags)
+      @glib.__send__(:merge_back_cflags, cflags)
+    end
+
+    def test_d
+      assert_equal(["-DFOO"],
+                   merge_back_cflags(["-DFOO", "-DFOO"]))
+    end
+
+    def test_w
+      assert_equal(["-Wno-unknown-warning-option"],
+                   merge_back_cflags(["-Wno-unknown-warning-option",
+                                      "-Wno-unknown-warning-option"]))
+    end
+
+    def test_wa_wl_wp
+      assert_equal(["-Wa,--noexecstack", "-Wl,--as-needed", "-Wp,-DFOO",
+                    "-Wa,--noexecstack", "-Wl,--as-needed", "-Wp,-DFOO"],
+                   merge_back_cflags(["-Wa,--noexecstack", "-Wl,--as-needed", "-Wp,-DFOO",
+                                      "-Wa,--noexecstack", "-Wl,--as-needed", "-Wp,-DFOO"]))
+    end
+
+    def test_mixed
+      assert_equal(["-Wl,--as-needed", "-DFOO", "-Wall", "-Wl,--as-needed"],
+                   merge_back_cflags(["-DFOO", "-Wall", "-Wl,--as-needed",
+                                      "-DFOO", "-Wall", "-Wl,--as-needed"]))
+    end
+  end
 end