Fix CMAKE_HOST_SYSTEM_PROCESSOR detection on GNU/Hurd (#15147)

On Debian GNU/Hurd 'uname -p' prints "unknown", so use 'uname -m'
instead.
Help: Document the WINCE variable
Help: Document that the CHECK_* macros create cache variables

Otherwise callers may expect to be able to re-use result variables.
Merge topic 'fix-gnu-hurd-host-detection'

89d7a5a4 Fix CMAKE_HOST_SYSTEM_PROCESSOR detection on GNU/Hurd (#15147)
Merge topic 'doc-WINCE-var'

4b4555de Help: Document the WINCE variable
Merge topic 'doc-check-results-cached'

908c7439 Help: Document that the CHECK_* macros create cache variables
If: Extract cmConditionEvaluator from if() implementation
If: Introduce policy CMP0054 - don't dereference quoted variables in if()
Modules/Test*.cmake: Use if(DEFINED) to simplify conditions

Replace old hacks of the form 'if("${VAR}" MATCHES "^${VAR}$")'
with the much simpler 'if(NOT DEFINED ${VAR})'.
FindGTK2: Avoid depending on if() to dereference a quoted variable

Explicitly dereference GTK2_${_var}CONFIG_INCLUDE_DIR and
GTK2_${_var}_INCLUDE_DIR when comparing their values.
libarchive: Avoid depending on if() to dereference a quoted variable
cmcurl: Use if(DEFINED) to simplify conditions

Replace old hacks of the form 'if("${VAR}" MATCHES "^${VAR}$")'
with the much simpler 'if(NOT DEFINED ${VAR})'.
FPHSA: Avoid if() dereferencing of quoted variable

Legacy invocations may pass a variable name where "DEFAULT_MSG" belongs.
When comparing FPHSA_FAIL_MESSAGE to "DEFAULT_MSG", use a leading "x" on
both sides to avoid mistaking the value of the message for a variable
name.
Fix if() checks of CMAKE_SYSTEM_NAME on Cygwin

The CMAKE_SYSTEM_NAME is "CYGWIN", but we also define a variable
named "CYGWIN" to "1".  Avoid allowing if() to expand the "CYGWIN"
string as a variable.
CMake Nightly Date Stamp
Help: Fix set_tests_properties documentation typo

The example for FAIL_REGULAR_EXPRESSION should actually name it.
CMake Nightly Date Stamp
CMake Nightly Date Stamp
CMake Nightly Date Stamp
ExternalProject: Fix download retry logic

Do not check file's hash in download script.  If hash will not match
command ``file(DOWNLOAD ...)`` will fail with FATAL_ERROR, ``cmake -P``
will exit with unsuccessful code, and the build will stop.  Leave hash
checking to the existing implementation in the verify step.