Fix a CMake warning on release builds

It looks like this isn't a regression - all previous releases had this
issue. We simply didn't use the "-Werror=dev" flag before when testing
CMake. If the "pcre2_prerelease" variable was set to empty string, it
would be treated as "falsy" by CMake and not substituted correctly.

Author: NWilson

CMakeLists.txt

@@ -733,10 +733,9 @@ set(
 foreach(configure_line ${configure_lines})
   foreach(substitution_variable ${SEARCHED_VARIABLES})
     string(TOUPPER ${substitution_variable} substitution_variable_upper)
-    if(NOT ${substitution_variable_upper})
-      string(REGEX MATCH "m4_define\\(${substitution_variable}, *\\[(.*)\\]" MATCHED_STRING ${configure_line})
-      if(CMAKE_MATCH_1)
-        set(${substitution_variable_upper} ${CMAKE_MATCH_1})
+    if(NOT DEFINED ${substitution_variable_upper})
+      if("${configure_line}" MATCHES "m4_define\\(${substitution_variable}, *\\[(.*)\\]")
+        set(${substitution_variable_upper} "${CMAKE_MATCH_1}")
       endif()
     endif()
   endforeach()