build: Add support for non-source-directory builds.

build: Add support for non-source-directory builds.

Such as:

     mkdir build
     cd build
     ../configure
     make

This is implemented by having the configure script set a srcdir
variable in Makefile.config, and then sprinkling $(srcdir) into
various make rules. We also use vpath directives to convince GNU make
to find the source files from the original source directory.