Avoid using cmake glob vars if we are a subproject (#1459)

If jsoncpp is a subproject (like a git submodule), setting the
global cmake variables affect the entire project (changes the
structure of the output folders) and these changes prevent it.

CMakeLists.txt

@@ -96,10 +96,12 @@ option(BUILD_OBJECT_LIBS "Build jsoncpp_lib as a object library." ON)
 # Adhere to GNU filesystem layout conventions
 include(GNUInstallDirs)
 
-set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/lib" CACHE PATH "Archive output dir.")
-set(CMAKE_LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/lib" CACHE PATH "Library output dir.")
-set(CMAKE_PDB_OUTPUT_DIRECTORY     "${CMAKE_BINARY_DIR}/bin" CACHE PATH "PDB (MSVC debug symbol)output dir.")
-set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/bin" CACHE PATH "Executable/dll output dir.")
+if(CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
+    set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/lib" CACHE PATH "Archive output dir.")
+    set(CMAKE_LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/lib" CACHE PATH "Library output dir.")
+    set(CMAKE_PDB_OUTPUT_DIRECTORY     "${CMAKE_BINARY_DIR}/bin" CACHE PATH "PDB (MSVC debug symbol)output dir.")
+    set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/bin" CACHE PATH "Executable/dll output dir.")
+endif()
 
 set(JSONCPP_USE_SECURE_MEMORY "0" CACHE STRING "-D...=1 to use memory-wiping allocator for STL")