The INSTALL_INTERFACE include directory was written as
$<INSTALL_PREFIX>/${CMAKE_INSTALL_INCLUDEDIR}, which unconditionally
prepends the install prefix. When CMAKE_INSTALL_INCLUDEDIR is an
absolute path, the exported GTestTargets.cmake ends up with an invalid
doubled path such as "/usr/local//usr/local/include", and find_package
consumers fail at configure time with "Imported target includes
non-existent path".
Relative paths inside $<INSTALL_INTERFACE:...> are already interpreted
relative to the installation prefix, so the explicit $<INSTALL_PREFIX>/
prefix is redundant for relative values and wrong for absolute ones.
Dropping it keeps the exported path identical for relative values
(${_IMPORT_PREFIX}/include) and uses absolute values as-is.
Fixes#4817
