cmake: do influence CMAKE_DEBUG_POSTFIX of the outer project (if any)

Consider the following example:

    $ touch lib.c
    $ cat > CMakeLists.txt <<EOL
    add_subdirectory(/src/le/libevent libevent EXCLUDE_FROM_ALL)
    add_library(lib lib.c)
    EOL
    $ mkdir .cmake
    $ cmake -DCMAKE_BUILD_TYPE=debug -G Ninja ..
    $ ninja
    $ ls *.a
    liblibd.a

So now outer project also has "d" prefix for libraries.

Let's avoid this by setting non-cached variable.

Fixes: libevent#1371

CMakeLists.txt

@@ -37,7 +37,7 @@ if(NOT CMAKE_BUILD_TYPE)
 endif()
 string(TOLOWER "${CMAKE_BUILD_TYPE}" CMAKE_BUILD_TYPE_LOWER)
 
-set(CMAKE_DEBUG_POSTFIX d CACHE STRING "Debug library postfix" FORCE)
+set(CMAKE_DEBUG_POSTFIX d)
 
 set(EVENT__LIBRARY_TYPE DEFAULT CACHE STRING
     "Set library type to SHARED/STATIC/BOTH (default SHARED for MSVC, otherwise BOTH)")