FPHSA: Check _FOUND var name with STREQUAL

This change creates variables _FOUND_VAR_UPPER and _FOUND_VAR_MIXED to hold the constructed variable names, then compares them to FPHSA_FOUND_VAR using STREQUAL, instead of MATCHES.

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.)

e.g. consider a find module called FindUnitTest++.cmake. With the previous code, this FPHSA call:

find_package_handle_standard_args(UnitTest++ DEFAULT_MSG
  UnitTest++_LIBRARY UnitTest++_INCLUDE_DIR)

will work fine. But this one:

find_package_handle_standard_args(UnitTest++
  FOUND_VAR UnitTest++_FOUND
  REQUIRED_VARS UnitTest++_LIBRARY UnitTest++_INCLUDE_DIR)

will result in a regexp error:

$ cmake -P ./FindUnitTest++.cmake
RegularExpression::compile(): Nested *?+.
RegularExpression::compile(): Error in compile.
CMake Error at /usr/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:242 (if):
  if given arguments:

    "FPHSA_FOUND_VAR" "MATCHES" "^UnitTest++_FOUND\$" "OR" "FPHSA_FOUND_VAR" "MATCHES" "^UNITTEST++_FOUND\$"

  Regular expression "^UnitTest++_FOUND$" cannot compile
Call Stack (most recent call first):
  FindUnitTest++.cmake:61 (find_package_handle_standard_args)

The modified code handles the FOUND_VAR name correctly:

$ cmake -P ./FindUnitTest++.cmake
-- Found UnitTest++: /usr/lib64/libUnitTest++.so