%{prefix} macro added.

- It is used in automated building instructions.
	- It is used in auto-splits.
	- It is preset before reading the recipe to the first of the
	  default prefixes defined in this context, and can be reset
	  to force the recipe to build somewhere else.

src/context.cr

@@ -71,6 +71,8 @@ class Package::Context
 	# but it’ll be kind of enough for now.
 	property architecture = "x86_64"
 
+	property prefixes = ["/usr", "/", "/usr/weirdos"]
+
 	def initialize
 		@packaging_backends << Backend::Packaging.new "pkgutils" do |context, package|
 			puts "#{package.fake_root_directory} -> #{packages_directory}/#{package.name}##{package.version}-#{package.release}.pkg.tar.xz"
@@ -111,7 +113,7 @@ class Package::Context
 				next BuildStatus::Pass
 			end
 
-			child = context.sh "./configure --prefix=/usr #{recipe.options["configure"]? || ""}"
+			child = context.sh "./configure --prefix=#{recipe.prefix} #{recipe.options["configure"]? || ""}"
 
 			if child.exit_status == 0
 				BuildStatus::Success
@@ -154,19 +156,24 @@ class Package::Context
 
 		@splitter_backends << Backend::Splitter.new do |recipe|
 			Package.new(recipe, true).tap do |split|
+				prefixes = (@prefixes + [recipe.prefix]).uniq
+
 				split.name = "#{recipe.name}-man"
-				split.files = ["/usr/share/man"]
+				split.files = prefixes.map do |prefix|
+					"#{prefix}/share/man"
+				end
 			end
 		end
 
 		@splitter_backends << Backend::Splitter.new do |recipe|
 			Package.new(recipe, true).tap do |split|
+				prefixes = (@prefixes + [recipe.prefix]).uniq
+
 				split.name = "#{recipe.name}-dev"
-				split.files = ["/usr/include"]
-				split.file_patterns = [
-					Regex.new("^/lib/.*\\.a$"),
-					Regex.new("^/usr/lib/.*\\.a$")
-				]
+				split.files = prefixes.map { |x| "#{x}/include" }
+				split.file_patterns = prefixes.map do |prefix|
+					Regex.new("^" + prefix + ".*\\.a$")
+				end
 			end
 		end
 	end

src/recipe.cr

@@ -57,6 +57,8 @@ class Package::Recipe
 	getter sources       = Sources.new
 	getter packages      = Array(Package).new
 
+	@prefix : String?
+
 	property recipe_directory = "."
 
 	@working_uuid        : UUID = UUID.random
@@ -74,7 +76,8 @@ class Package::Recipe
 
 	def initialize(@context, filename : String)
 		specs = SpecFileParser.parse filename, {
-			"pkg" => fake_root_directory
+			"pkg" => fake_root_directory,
+			"prefix" => prefix
 		}
 
 		name : String? = nil
@@ -112,6 +115,8 @@ class Package::Recipe
 				@instructions.install << value.as_s_or_ls
 			when "dirname"
 				@dirname = value.as_s
+			when "prefix"
+				@prefix = value.as_s
 			when "dependencies"
 				value.as_a_or_s.each do |atom|
 					@run_dependencies << atom
@@ -182,6 +187,10 @@ class Package::Recipe
 		@dirname || "#{name}-#{version}"
 	end
 
+	def prefix : String
+		@prefix || @context.prefixes[0]? || "/usr"
+	end
+
 	def download
 		sources.each do |url|
 			next if url.scheme == "file"