cmake/modules/Findpmem: always set pmem_VERSION_STRING

cmake/modules/Findpmem: always set pmem_VERSION_STRING

before this change, `pmem_VERSION_STRING` is not set if it is not able
to fulfill the specified version requirement. the intention was to check
if the version is able to satisfy the requirement. but actually, passing
an empty `pmem_VERSION_STRING` to `find_package_handle_standard_args()`
as the option of `VERSION_VAR` does not fail this check. on the
contrary, it prints

-- Found pmem: pmem_pmemobj_INCLUDE_DIR;pmem_pmem_INCLUDE_DIR (Required
is at least version "1.17")

if we requires pmem 1.17, while the found version is, for instance,
1.10.

if the required version is 1.7, and the found version is 1.10, the
output from cmake is:

-- Found pmem: pmem_pmemobj_INCLUDE_DIR;pmem_pmem_INCLUDE_DIR (found
suitable version "1.10", minimum required is "1.7")

in this change, the version spec is not specified when calling
`pkg_check_modules()`. so, `PKG_${component}_VERSION` is always set.
and we can always delegate the version checking to
`find_package_handle_standard_args()`. please note, we use the lower
version returned by pkg-config if multiple components are required and
both pkg-config settings return their versions.

Signed-off-by: Kefu Chai <kchai@redhat.com>