Add support for disabling features at build time

Signed-off-by: Izuru Yakumo <yakumo.izuru@chaotic.ninja>

git-svn-id: https://svn.yakumo.dev/yakumo.izuru/aya/trunk@87 cec141ff-132a-4243-88a5-ce187bd62f94

Makefile

@@ -4,7 +4,6 @@ PREFIX ?= /usr/local
 DATE ?= `date -u +%F`
 GOOS ?= `go env GOOS`
 VERSION ?= `git describe --tags`
-
 build:
 	go build ${GOFLAGS} ./cmd/aya
 clean:

README.md

@@ -15,7 +15,7 @@ Named after [Aya Shameimaru](https://en.touhouwiki.net/wiki/Aya_Shameimaru) from
 
 ## Installation
 
-Build it manually assuming you have Go (>=1.17) installed:
+Build it manually provided you have Go (>=1.17) installed:
 
 	$ go install marisa.chaotic.ninja/aya/cmd/aya@latest (1)
 	--- or ---
@@ -27,6 +27,10 @@ Build it manually assuming you have Go (>=1.17) installed:
 (1) If you use this method, the `aya version` subcommand may print the wrong string,
 but it should not be a problem unless you use it on a page.
 
+You can also disable certain features at build time, with the `-tags` switch.
+Currently, these tags are available: `noamber`, `nogcss`.
+See `go help buildconstraint` for more details.
+
 ## Ideology
 
 Keep your texts in markdown, [amber](https://github.com/eknkc/amber), or html format right in the main directory
@@ -35,7 +39,7 @@ of your blog/site.
 Keep all service files (extensions, layout pages, deployment scripts etc)
 in the `.aya` subdirectory.
 
-Define variables in the header of the content files using [YAML](https://www.yaml.io) :
+Define variables in the header of the content files using [YAML](https://noyaml.com) :
 
 ```markdown
 title: My web site

cmd/aya/amber.go

@@ -1,3 +1,4 @@
+//go:build !noamber
 // Render .amber files into .html
 package main
 

cmd/aya/amber_stub.go

@@ -0,0 +1,11 @@
+//go:build noamber
+package main
+
+import (
+	"errors"
+	"io"
+)
+
+func buildAmber(path string, w io.Writer, vars Vars) error {
+	return errors.New("Amber support was disabled on build-time")
+}

cmd/aya/gcss.go

@@ -1,3 +1,4 @@
+//go:build !nogcss
 // Render .gcss files into .css
 package main
 

cmd/aya/gcss_stub.go

@@ -0,0 +1,11 @@
+//go:build nogcss
+package main
+
+import (
+	"errors"
+	"io"
+)
+
+func buildGCSS(path string, w io.Writer) error {
+	return errors.New("GCSS support was disabled at build time")
+}