Clarify need to specify --prefix with depends

If not cross compiling, it might actually be nice for it to be picked up
automatically. But for now clarify the readme to try to minimize confusion.

chaincodelabs/libmultiprocess#4 (comment)
chaincodelabs/libmultiprocess#5 (comment)

depends/README.md

@@ -12,14 +12,17 @@ For example:
 
     make HOST=x86_64-w64-mingw32 -j4
 
-A prefix will be generated that's suitable for plugging into Bitcoin's
-configure. In the above example, a dir named x86_64-w64-mingw32 will be
+**Bitcoin's configure script by default will ignore the depends output.** In
+order for it to pick up libraries, tools, and settings from the depends build,
+you must point it at the appropriate `--prefix` directory generated by the
+build. In the above example, a prefix dir named x86_64-w64-mingw32 will be
 created. To use it for Bitcoin:
 
     ./configure --prefix=`pwd`/depends/x86_64-w64-mingw32
 
 Common `host-platform-triplets` for cross compilation are:
 
+- `x86_64-pc-linux-gnu` for x86 Linux
 - `x86_64-w64-mingw32` for Win64
 - `x86_64-apple-darwin14` for macOS
 - `arm-linux-gnueabihf` for Linux ARM 32 bit