Unquote pkg-config output if they are printed within quotes. pkgconf
<1.5.1 and classic pkg-config <0.29.1 prints quoted variables without
unquoting them, this breaks returning variables with multiple values
as a list behavior of CMake.

Add a new test case for pkg_get_variable with multiple values to test
list behavior and backslash escaped spaces within variable values.

Fixes: #25904