Adopt Linux-like .so versioning at install-time.

Details:
- Changed the naming conventions used for installed libraries and
  symlinks to more closely mirror patterns used by typical GNU/Linux
  libraries. Whereas previously static and shared libraries were
  installed and symlinked as follows:

    (library) libblis-0.3.2-15-haswell.a
    (library) libblis-0.3.2-15-haswell.so
    (symlink) libblis.a -> libblis-0.3.2-15-haswell.a
    (symlink) libblis.so -> libblis-0.3.2-15-haswell.so

  we now use the following naming conventions:

    (library) libblis.a
    (symlink) libblis.so -> libblis.so.0.1.2
    (symlink) libblis.so.0 -> libblis.so.0.1.2
    (library) libblis.so.0.1.2

  where 0.1.2 indicates shared library major, minor, and build versions
  of 0, 1, and 2, respectively. The conventional version string can
  still be queried by linking to the library in question and then calling
  bli_info_get_version_str(). (The testsuite binary does this
  automatically at startup.)
- Added logic to common.mk to set the soname field in the shared library
  via the -soname linker flag.
- Added a 'so_version' file to the top-level directory containing two
  lines. The first line specifies the .so major version number, and the
  second line specifies the minor and build version numbers joined with
  a '.'. This file is read by configure and those values substituted
  into build/config.mk.in to define SO_MAJOR, SO_MINORB, and SO_MMB
  variables.

so_version

@@ -0,0 +1,2 @@
+0
+0.0