ExternalProject: Allow generator expressions with LOG_* options (#15287)

Use file(GENERATE) to write the logging wrapper scripts to evaluate
generator expressions.  Use a per-config script names in case the
content varies by configuration.

Tests/ExternalProjectSubdir/CMakeLists.txt

@@ -6,7 +6,7 @@ ExternalProject_Add(Subdir1
   SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/Subdir1
   BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/Subdir1
 
-  CMAKE_ARGS -DNORMAL_VAR=NORMAL_VALUE
+  CMAKE_ARGS -DNORMAL_VAR=NORMAL_VALUE -DGENEX_VAR=$<1:GENEX_VALUE>
   LOG_CONFIGURE 1
 
   BUILD_COMMAND ""

Tests/ExternalProjectSubdir/Subdir1/CMakeLists.txt

@@ -4,3 +4,7 @@ project(Subdir1 NONE)
 if(NOT "${NORMAL_VAR}" STREQUAL "NORMAL_VALUE")
   message(SEND_ERROR "NORMAL_VAR != 'NORMAL_VALUE'")
 endif()
+
+if(NOT "${GENEX_VAR}" STREQUAL "GENEX_VALUE")
+  message(SEND_ERROR "GENEX_VAR != 'GENEX_VALUE'")
+endif()