CMP0053: Fix double warning on uninitialized variables in -P mode

When `CMP0053` is not set to OLD or NEW then we compute both variants
in case we need to warn about a behavior change.  Do not allow both
code paths to produce an uninitialized variable warning.

Fixes: #18552

Source/cmMakefile.cxx

@@ -2766,7 +2766,7 @@ cmake::MessageType cmMakefile::ExpandVariablesInStringNew(
             } else {
               varresult = value;
             }
-          } else if (!removeEmpty) {
+          } else if (!removeEmpty && !this->SuppressSideEffects) {
             // check to see if we need to print a warning
             // if strict mode is on and the variable has
             // not been "cleared"/initialized with a set(foo ) call

Tests/RunCMake/CommandLine/RunCMakeTest.cmake

@@ -348,6 +348,10 @@ set(RunCMake_TEST_OPTIONS --trace-expand --warn-uninitialized)
 run_cmake(trace-expand-warn-uninitialized)
 unset(RunCMake_TEST_OPTIONS)
 
+set(RunCMake_TEST_OPTIONS --warn-uninitialized)
+run_cmake(warn-uninitialized)
+unset(RunCMake_TEST_OPTIONS)
+
 set(RunCMake_TEST_OPTIONS --trace-source=trace-only-this-file.cmake)
 run_cmake(trace-source)
 unset(RunCMake_TEST_OPTIONS)

Tests/RunCMake/CommandLine/warn-uninitialized-stderr.txt

@@ -0,0 +1,5 @@
+^CMake Warning \(dev\) at warn-uninitialized.cmake:1 \(set\):
+  uninitialized variable 'WARN_FROM_NORMAL_CMAKE_FILE'
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.$

Tests/RunCMake/CommandLine/warn-uninitialized.cmake

@@ -0,0 +1 @@
+set(FOO "${WARN_FROM_NORMAL_CMAKE_FILE}")