cmake: Fix fallback version number

OpusFilePackageVersion falls back to returning version 0 if
it cannot find an actual version number from git or a release.

However, CMakeLists.txt assumes the package version is at
least a two part x.y version number and so fails trying
to parse out the minor revision number when run from a
bare directory:

    Opus Project version: 0
    CMake Error at CMakeLists.txt:9 (list):
      list index: 1 out of range (-1, 0)

Instead, return 0.0 as the project version. Addresses an
issue raised in #54

Author: rillian

cmake/OpusFilePackageVersion.cmake

@@ -60,7 +60,7 @@ function(get_package_version PACKAGE_VERSION PROJECT_VERSION)
     # fail to parse version from git and package version
     message(WARNING "Could not get package version.")
     set(OPUSFILE_PACKAGE_VERSION 0)
-    set(OPUSFILE_PROJECT_VERSION 0)
+    set(OPUSFILE_PROJECT_VERSION 0.0)
   endif()
 
   message(STATUS "Opus project version: ${OPUSFILE_PROJECT_VERSION}")