Add variable for doing out-of-tree builds.

Normally we run ./configure from within the build tree, but autotools
now allows doing out-of-tree builds by running configure while cd'ed
to a different directory. Add a config variable `AUTOTOOLS_BUILD_PATH`
to allow this on a per-package basis, and set this for the SDL2
package; SDL2 hg complains when doing in-tree builds because certain
files get clobbered.

chocpkg/modules/build_autotools.sh

@@ -34,7 +34,13 @@ do_build() {
         autoreconf -fi
     fi
 
-    ./configure --prefix="$PACKAGE_INSTALL_DIR" $host_opt \
+    local configure_path="$PWD/configure"
+    if [ ! -z "${AUTOTOOLS_BUILD_PATH:-}" ]; then
+        mkdir -p "$AUTOTOOLS_BUILD_PATH"
+        cd "$AUTOTOOLS_BUILD_PATH"
+    fi
+
+    "$configure_path" --prefix="$PACKAGE_INSTALL_DIR" $host_opt \
                 $PACKAGE_CONFIGURE_OPTS || (
         error_exit "Failed to configure package $PACKAGE_NAME for build."
     )
@@ -47,6 +53,10 @@ do_build() {
 # Function encapsulating the "make install" step which should work for
 # most autotools-based packages, but can be overridden by packages.
 do_install() {
+    if [ ! -z "${AUTOTOOLS_BUILD_PATH:-}" ]; then
+        cd "$AUTOTOOLS_BUILD_PATH"
+    fi
+
     make install || (
         error_exit "Failed to install package $PACKAGE_NAME."
     )

pkgdef/SDL2.sh

@@ -20,4 +20,7 @@ if [ $(uname) = "Cygwin" ] || [[ "$BUILD_HOST" = *mingw* ]]; then
     config_options+=" --disable-directx"
 fi
 
+# For SDL, we do an out-of-tree build in a subdirectory, since the configure
+# script can complain otherwise.
+AUTOTOOLS_BUILD_PATH=build-artifacts
 build_autotools $config_options