Use the TAGLIB_DIR env. var. to use non-standard taglib prefix

If the TAGLIB_DIR env. var. is not specified, taglib is assumed to be
installed at standard locations. Otherwise, it is assumed that taglib
headers are located at $TAGLIB_DIR/include and taglib libraries at
$TAGLIB_DIR/lib. This prefix is taken into account by both the swig and
compile rake tasks.

Author: Sigill

README.md

@@ -95,6 +95,12 @@ Build and install gem into system gems:
 
     rake install
 
+The `swig` and `compile` rake tasks can use the `TAGLIB_DIR` environment
+variable to build against non-standard taglib installations. It is assumed
+that taglib headers are located at `$TAGLIB_DIR/include` and taglib libraries
+at `$TAGLIB_DIR/lib`. To run taglib-ruby with non-standard taglib
+installations, use the `LD_LIBRARY_PATH` env. variable.
+
 ### Workflow
 
 * Check out the latest master to make sure the feature hasn't been

ext/extconf_common.rb

@@ -20,7 +20,14 @@ def error msg
   abort
 end
 
-dir_config('tag')
+if ENV.has_key?('TAGLIB_DIR') and !File.directory?(ENV['TAGLIB_DIR'])
+  error "When defined, the TAGLIB_DIR environment variable must point to a valid directory."
+end
+
+# If specified, use the TAGLIB_DIR environment variable as the prefix
+# for finding taglib headers and libs. See MakeMakefile#dir_config
+# for more details.
+dir_config('tag', (ENV['TAGLIB_DIR'] if ENV.has_key?('TAGLIB_DIR')))
 
 # When compiling statically, -lstdc++ would make the resulting .so to
 # have a dependency on an external libstdc++ instead of the static one.

tasks/swig.rake

@@ -1,11 +1,30 @@
 # Tasks for generating SWIG wrappers in ext
 
+# Execute SWIG for the specified extension.
+# Arguments:
+# mod:: The name of the SWIG wrapper to process.
+#
+# If the TAGLIB_DIR environment variable points to a directory,
+# $TAGLIB_DIR/include will be searched first for taglib headers.
 def run_swig(mod)
   swig = `which swig`.chomp
   if swig.empty?
     swig = `which swig2.0`.chomp
   end
-  sh "cd ext/#{mod} && #{swig} -c++ -ruby -autorename -initname #{mod} -I/usr/local/include -I/usr/include #{mod}.i"
+
+  # Standard search location for headers
+  include_args = %w{-I/usr/local/include -I/usr/include}
+
+  if ENV.has_key?('TAGLIB_DIR')
+    unless File.directory?(ENV['TAGLIB_DIR'])
+      abort "When defined, the TAGLIB_DIR environment variable must point to a valid directory."
+    end
+
+    # Push it in front to get it searched first.
+    include_args.unshift('-I' + ENV['TAGLIB_DIR'] + '/include')
+  end
+
+  sh "cd ext/#{mod} && #{swig} -c++ -ruby -autorename -initname #{mod} #{include_args.join(' ')} #{mod}.i"
 end
 
 task :swig =>