cmMakefile: Fix compilation on EDG-based compilers such as LCC

Compilers based on EDG frontend sometimes throw
an internal error while using `this->` at some
circumstances. While it is up to be fixed in future
versions of front end, this bug still occurs in
some modern compilers, such as LCC for Elbrus CPUs,
and probably others (maybe ICC). It caused CMake to be
unbuildable by these compilers. This patch fixes it.

Source/cmMakefile.cxx

diff --git a/Source/cmMakefile.cxx b/Source/cmMakefile.cxx
index a490dac..83984f7 100644
--- a/Source/cmMakefile.cxx
+++ b/Source/cmMakefile.cxx
@@ -3982,11 +3982,10 @@
   if (prop == "TESTS") {
     std::vector<std::string> keys;
     // get list of keys
-    std::transform(this->Tests.begin(), this->Tests.end(),
-                   std::back_inserter(keys),
-                   [](decltype(this->Tests)::value_type const& pair) {
-                     return pair.first;
-                   });
+    const auto* t = this;
+    std::transform(
+      t->Tests.begin(), t->Tests.end(), std::back_inserter(keys),
+      [](decltype(t->Tests)::value_type const& pair) { return pair.first; });
     output = cmJoin(keys, ";");
     return cmValue(output);
   }