Make package installation on Linux optional

Xamarin.Android needs a number of packages to be installed on Linux
in order to build properly. The makefile provides support for that but
it requires using `sudo` to invoke the package installer. That may not
be desired on build bots and so this commit makes package installation
optional. In order to avoid package installation invoke make like so:

   make NO_SUDO=true prepare

In this mode the makefile will merely report the installation status of
all the dependency packages instead of installing them.

Author: grendello

Makefile

@@ -44,6 +44,8 @@ include build-tools/scripts/BuildEverything.mk
 
 # Please keep the package names sorted
 ifeq ($(OS),Linux)
+NO_SUDO ?= false
+
 UBUNTU_DEPS          = \
 	autoconf \
 	autotools-dev \
@@ -84,10 +86,34 @@ prepare:: linux-prepare-$(LINUX_DISTRO) linux-prepare-$(LINUX_DISTRO)-$(LINUX_DI
 		cat Documentation/binfmt_misc-warning-Linux.txt ; \
 	fi
 
+ifeq ($(NO_SUDO),false)
 linux-prepare-Ubuntu::
+	@echo
 	@echo Installing build depedencies for $(LINUX_DISTRO)
 	@echo Will use sudo, please provide your password as needed
+  @echo
 	sudo apt-get -f -u install $(UBUNTU_DEPS)
+else
+linux-prepare-Ubuntu::
+	@echo
+	@echo sudo is disabled, cannot install dependencies
+	@echo Listing status of all the dependencies
+	@PACKAGES_MISSING=no ; \
+	for p in $(UBUNTU_DEPS); do \
+	   if dpkg -l $$p > /dev/null 2>&1 ; then \
+	     echo "[INSTALLED] $$p" ; \
+	   else \
+	     echo "[ MISSING ] $$p" ; \
+	     PACKAGES_MISSING=yes ; \
+	   fi ; \
+	done ; \
+	echo ; \
+	if [ "x$$PACKAGES_MISSING" = "xyes" ]; then \
+	  echo Some packages are missing, cannot continue ; \
+	  echo ; \
+	  false ; \
+	fi
+endif
 
 linux-prepare-$(LINUX_DISTRO)::