Doc: Always use absolute paths

doc/build-unix.md

@@ -2,6 +2,16 @@ UNIX BUILD NOTES
 ====================
 Some notes on how to build Bitcoin in Unix. 
 
+Note
+---------------------
+Always use absolute paths to configure and compile bitcoin and the dependencies,
+for example, when specifying the the path of the dependency:
+
+	../dist/configure --enable-cxx --disable-shared --with-pic --prefix=$BDB_PREFIX
+
+Here BDB_PREFIX must absolute path - it is defined using $(pwd) which ensures
+the usage of the absolute path.
+
 To Build
 ---------------------