Simplify instructions about choosing the installation prefix

Clarify the README thanks to the use of pkg-config, and add a
configure-time warning so people notice about the problem without having
to read the README.

README

@@ -21,14 +21,10 @@ Building & Installing Gtk#:
     to make it so mono (and mint) can find your assemblies.  In other words,
     doing something like:
 
-        ./configure --prefix=/the/path/that/was/used/for/mono
+        ./configure --prefix=`pkg-config --variable=prefix mono`
         make
         make install
 
-    (Of course, replace "/the/path/that/was/used/for/mono" with whatever path
-    which was used for Mono.  This might have been "/usr", "/usr/local", or
-    something similar.)
-
     If you are compiling from GIT, you will need libtool and the auto* tools
     and will need to replace the configure above with autogen.sh.
 

configure.ac

@@ -292,3 +292,10 @@ if test "x$enable_monodoc" = "xyes" -a "x$doc_sources_dir" != "x$prefix/lib/mono
 fi
 echo "---"
 
+mono_prefix=`pkg-config --variable=prefix mono`
+if test "x$mono_prefix" != "x$prefix"; then
+        AC_WARN(Prefix to use ($prefix) is not the same as Mono's ($mono_prefix). Consider using
+		    ./configure --prefix=$mono_prefix
+		    See the README for more information.
+	)
+fi