Before this patch, the write_basic_package_version_file() function of
the CMakePackageConfigHelpers module always emitted an architecture
check, even if the ARCH_INDEPENDENT option was specified. While this is
not an issue when configuring builds, as the check is skipped, this can
create issues when the "arch independent" version files are installed in
the datadir (e.g. /usr/share) in a MultiArch environment like Debian,
where different architecture packages of the same libraries can be
coinstalled; as the amd64 version of a given library contains "8 * 8"
in the file, while the i386 one contains "4 * 8", there's a conflict, as
files in /usr/share are expected to be identical across architectures.

This patch fixes this issue by only emitting the architecture check code
if needed; when ARCH_INDEPENDENT is specified, no code is written at
all.

Here's a diff between the version files generated before and after this
patch:

    diff -u old/indep.cmake...