Rez 2.13.0 errors when package descriptions span more than one line

[ttanimura]

We have a package that has this in its package.py:

description = \
    """
    The spMathNodes module is a tool to easily create and connect math related
    utility nodes in Maya using Python language features. This will make
    writing and reading code that builds these node networks much easier. The
    scope and goal of this module is to provide basic math operation including
    support for vectors and matrices.
    """

When we build this package with rez-build --verbose, it fails with:

Unmatched ".
CMAKE_MODULE_PATH: Undefined variable.
14:50:07 ERROR    BuildError: The cmake build system failed.

Changing the description to be a single line fixes the issue.

[JeanChristopheMorinPerso]

#797, #937, #964, #1107