FPHSA: Check _FOUND var name with STREQUAL

Using MATCHES "^${_Name}_FOUND$" limits the name of the found variable,
since it can't contain any regular expression special-chars (period,
plus sign, etc.)
diff --git a/Modules/FindPackageHandleStandardArgs.cmake b/Modules/FindPackageHandleStandardArgs.cmake
index a7c3eae..a078049 100644
--- a/Modules/FindPackageHandleStandardArgs.cmake
+++ b/Modules/FindPackageHandleStandardArgs.cmake
@@ -289,10 +289,12 @@
   string(TOLOWER ${_NAME} _NAME_LOWER)
 
   if(FPHSA_FOUND_VAR)
-    if(FPHSA_FOUND_VAR MATCHES "^${_NAME}_FOUND$"  OR  FPHSA_FOUND_VAR MATCHES "^${_NAME_UPPER}_FOUND$")
+    set(_FOUND_VAR_UPPER ${_NAME_UPPER}_FOUND)
+    set(_FOUND_VAR_MIXED ${_NAME}_FOUND)
+    if(FPHSA_FOUND_VAR STREQUAL _FOUND_VAR_MIXED  OR  FPHSA_FOUND_VAR STREQUAL _FOUND_VAR_UPPER)
       set(_FOUND_VAR ${FPHSA_FOUND_VAR})
     else()
-      message(FATAL_ERROR "The argument for FOUND_VAR is \"${FPHSA_FOUND_VAR}\", but only \"${_NAME}_FOUND\" and \"${_NAME_UPPER}_FOUND\" are valid names.")
+      message(FATAL_ERROR "The argument for FOUND_VAR is \"${FPHSA_FOUND_VAR}\", but only \"${_FOUND_VAR_MIXED}\" and \"${_FOUND_VAR_UPPER}\" are valid names.")
     endif()
   else()
     set(_FOUND_VAR ${_NAME_UPPER}_FOUND)