infra: Introduce BR2_HOST_NEEDS_JAVA

XBMC needs Java on the host in order to build, because it uses a
code-generator which is built in two phases: In the first phase SWIG is used
to parse C++ header files that define the API.  SWIG outputs an XML file
that contains a complete description of the structure of the API.  In the
second phase, the XML file is ingested by a Groovy (Java) program that then
creates C++ code that forms the bridge to the scripting language (Python).

The second phase is why we need java on the host.

You can learn more at the XBMC's wiki:
http://wiki.xbmc.org/index.php?title=Codegeneration#How_it_works

In order to check that, this patch introduce this mechanism in
dependencies.sh, and it also defines the variable in Config.in

[Peter: fix error message]
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Config.in

@@ -18,6 +18,11 @@ config BR2_EXTERNAL
 	string
 	option env="BR2_EXTERNAL"
 
+# Hidden boolean selected by packages in need of Java in order to build
+# (example: xbmc)
+config BR2_HOST_NEEDS_JAVA
+	bool
+
 # Hidden boolean selected by pre-built packages for x86, when they
 # need to run on x86-64 machines (example: pre-built external
 # toolchains, binary tools like SAM-BA, etc.).

support/dependencies/dependencies.sh

@@ -191,6 +191,14 @@ if grep -q ^BR2_PACKAGE_CLASSPATH=y $BR2_CONFIG ; then
 	done
 fi
 
+if grep -q ^BR2_HOST_NEEDS_JAVA=y $BR2_CONFIG ; then
+	if ! which java > /dev/null ; then
+		echo >&2
+		echo "You must install 'java' on your build machine" >&2
+		exit 1
+	fi
+fi
+
 if grep -q ^BR2_HOSTARCH_NEEDS_IA32_LIBS=y $BR2_CONFIG ; then
 	if test ! -f /lib/ld-linux.so.2 ; then
 		echo