Revert the use of string(APPEND) in .cmake.in files

The generated files may be consumed by older cmake versions which do not
support string(APPEND).

Modules/BasicConfigVersion-AnyNewerVersion.cmake.in

@@ -26,6 +26,6 @@ endif()
 # check that the installed version has the same 32/64bit-ness as the one which is currently searching:
 if(NOT CMAKE_SIZEOF_VOID_P STREQUAL "@CMAKE_SIZEOF_VOID_P@")
    math(EXPR installedBits "@CMAKE_SIZEOF_VOID_P@ * 8")
-   string(APPEND PACKAGE_VERSION " (${installedBits}bit)")
+   set(PACKAGE_VERSION "${PACKAGE_VERSION} (${installedBits}bit)")
    set(PACKAGE_VERSION_UNSUITABLE TRUE)
 endif()

Modules/BasicConfigVersion-ExactVersion.cmake.in

@@ -42,6 +42,6 @@ endif()
 # check that the installed version has the same 32/64bit-ness as the one which is currently searching:
 if(NOT CMAKE_SIZEOF_VOID_P STREQUAL "@CMAKE_SIZEOF_VOID_P@")
   math(EXPR installedBits "@CMAKE_SIZEOF_VOID_P@ * 8")
-  string(APPEND PACKAGE_VERSION " (${installedBits}bit)")
+  set(PACKAGE_VERSION "${PACKAGE_VERSION} (${installedBits}bit)")
   set(PACKAGE_VERSION_UNSUITABLE TRUE)
 endif()

Modules/BasicConfigVersion-SameMajorVersion.cmake.in

@@ -41,6 +41,6 @@ endif()
 # check that the installed version has the same 32/64bit-ness as the one which is currently searching:
 if(NOT CMAKE_SIZEOF_VOID_P STREQUAL "@CMAKE_SIZEOF_VOID_P@")
   math(EXPR installedBits "@CMAKE_SIZEOF_VOID_P@ * 8")
-  string(APPEND PACKAGE_VERSION " (${installedBits}bit)")
+  set(PACKAGE_VERSION "${PACKAGE_VERSION} (${installedBits}bit)")
   set(PACKAGE_VERSION_UNSUITABLE TRUE)
 endif()