mkmf.rb: -I flags to $INCFLAGS

* lib/mkmf.rb (MakeMakefile#pkg_config): separate -I flags to
  $INCFLAGS, which is used by CPP.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47458 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mkmf.rb

@@ -1759,22 +1759,37 @@ SRC
   def pkg_config(pkg, option=nil)
     if pkgconfig = with_config("#{pkg}-config") and find_executable0(pkgconfig)
       # iff package specific config command is given
-      get = proc {|opt| `#{pkgconfig} --#{opt}`.strip}
     elsif ($PKGCONFIG ||=
            (pkgconfig = with_config("pkg-config", ("pkg-config" unless CROSS_COMPILING))) &&
            find_executable0(pkgconfig) && pkgconfig) and
         system("#{$PKGCONFIG} --exists #{pkg}")
       # default to pkg-config command
-      get = proc {|opt| `#{$PKGCONFIG} --#{opt} #{pkg}`.strip}
+      pkgconfig = $PKGCONFIG
+      get = proc {|opt|
+        opt = IO.popen("#{$PKGCONFIG} --#{opt} #{pkg}", err:[:child, :out], &:read)
+        opt.strip if $?.success?
+      }
     elsif find_executable0(pkgconfig = "#{pkg}-config")
       # default to package specific config command, as a last resort.
-      get = proc {|opt| `#{pkgconfig} --#{opt}`.strip}
+    else
+      pkgconfig = nil
+    end
+    if pkgconfig
+      get ||= proc {|opt|
+        opt = IO.popen("#{pkgconfig} --#{opt}", err:[:child, :out], &:read)
+        opt.strip if $?.success?
+      }
     end
     orig_ldflags = $LDFLAGS
     if get and option
       get[option]
     elsif get and try_ldflags(ldflags = get['libs'])
+      if incflags = get['cflags-only-I']
+        $INCFLAGS << " " << incflags
+        cflags = get['cflags-only-other']
+      else
         cflags = get['cflags']
+      end
       libs = get['libs-only-l']
       ldflags = (Shellwords.shellwords(ldflags) - Shellwords.shellwords(libs)).quote.join(" ")
       $CFLAGS += " " << cflags