Add variants support.

It's nice to be able to build from source control HEAD, and
especially for packages under than Chocolate Doom itself. As a start,
define "latest" variants for all the SDL2 packages.

chocpkg/chocpkg

@@ -85,6 +85,14 @@ configure_for_package() {
         PACKAGE_TYPE=target
     fi
 
+    # Allow /suffix to specify a variant to build.
+    if [[ "$package" =~ .*/.* ]]; then
+        PACKAGE_VARIANT=${package/#*\//}
+        package=${package/%\/*/}
+    else
+        PACKAGE_VARIANT=stable
+    fi
+
     # Defaults before loading package file.
     PACKAGE_DESCRIPTION="Package $package"
     DEPENDENCIES=""

chocpkg/chocpkg_modules.sh

@@ -5,6 +5,13 @@ install_module() {
     ${module_name}::init "$@"
 }
 
+variant() {
+    local for_variant=$1; shift
+    if [ "$PACKAGE_VARIANT" = "$for_variant" ]; then
+        "$@"
+    fi
+}
+
 package_group() {
     install_module package_group "$@"
 }

pkgdef/README.md

@@ -75,6 +75,32 @@ Build modules specify how to build the package after it is fetched:
 build_autotools --disable-broken-feature
 ```
 
+### Variants
+
+Variants allow building of different versions of the same package. An
+example is "latest stable release" vs. "latest version from source
+repository". The default variant is called `stable`. A variant can be
+specified on the command line with a suffix, for example, to build the
+variant `latest` of package `neato-lib`:
+
+```shell
+chocpkg install neato-lib/latest
+```
+
+In package files the `variant` function is used for conditional code
+that is only executed for a particular variant. Usually this is used
+to select a fetch module. For example:
+
+```shell
+variant stable fetch_download http://example.com/neato-lib-0.0.1.tar.gz
+variant latest fetch_git git://example.com/neato-lib.git
+variant frob-branch fetch_git git://example.com/neato-lib.git frob-branch
+```
+
+By convention, the `stable` variant is "the most recent stable release
+of the package" while the `latest` variant is "the latest version in
+the source control repository".
+
 ### Other modules and functions
 
 * `dependencies`: Arguments provided to the function are the names

pkgdef/SDL2.sh

@@ -1,6 +1,8 @@
 description "Simple DirectMedia Layer"
 check_pkgconfig sdl2
-fetch_download http://www.libsdl.org/release/SDL2-2.0.4.tar.gz
+
+variant stable fetch_download http://www.libsdl.org/release/SDL2-2.0.4.tar.gz
+variant latest fetch_hg https://hg.libsdl.org/SDL/
 
 config_options=
 

pkgdef/SDL2_image.sh

@@ -1,7 +1,10 @@
 description "SDL imaging library"
 dependencies SDL2
 check_pkgconfig SDL2_image
-fetch_download https://www.libsdl.org/projects/SDL_image/release/SDL2_image-2.0.1.tar.gz
+
+variant stable fetch_download \
+    https://www.libsdl.org/projects/SDL_image/release/SDL2_image-2.0.1.tar.gz
+variant latest fetch_hg https://hg.libsdl.org/SDL_image/
 
 # SDL2_image supports lots of different file formats but we only care
 # about .png files. So just build a really thin library without support

pkgdef/SDL2_mixer.sh

@@ -1,7 +1,10 @@
 description "SDL audio mixing and music library"
 dependencies SDL2
 check_pkgconfig SDL2_mixer
-fetch_download http://www.libsdl.org/projects/SDL_mixer/release/SDL2_mixer-2.0.1.tar.gz
+
+variant stable fetch_download \
+    http://www.libsdl.org/projects/SDL_mixer/release/SDL2_mixer-2.0.1.tar.gz
+variant latest fetch_hg https://hg.libsdl.org/SDL_mixer
 
 # Disable dependencies on external libraries for sound file formats:
 config_options="

pkgdef/SDL2_net.sh

@@ -1,5 +1,7 @@
 description "SDL networking library"
 dependencies SDL2
 check_pkgconfig SDL2_net
-fetch_download http://www.libsdl.org/projects/SDL_net/release/SDL2_net-2.0.1.tar.gz
+variant stable fetch_download \
+    http://www.libsdl.org/projects/SDL_net/release/SDL2_net-2.0.1.tar.gz
+variant latest fetch_hg https://hg.libsdl.org/SDL_net
 build_autotools