BUG: New CMake features used that break backward compatibility

We desire for jsoncpp to compile and be readily available
with older  versions of cmake.  The use of newer cmake
commands requires conditional statements so that older
strategies can be used with older versions of cmake.

Resolves: open-source-parsers#1018

Author: hjmjohnson

Diff for: CMakeLists.txt

@@ -104,11 +104,23 @@ macro(UseCompilationWarningAsError)
     if(MSVC)
         # Only enabled in debug because some old versions of VS STL generate
         # warnings when compiled in release configuration.
-        add_compile_options($<$<CONFIG:Debug>:/WX>)
+        if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
+          add_compile_options($<$<CONFIG:Debug>:/WX>)
+        else()
+          set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} /WX ")
+        endif()
     elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
-        add_compile_options(-Werror)
+        if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
+          add_compile_options(-Werror)
+        else()
+          set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror")
+        endif()
         if(JSONCPP_WITH_STRICT_ISO)
+          if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
             add_compile_options(-pedantic-errors)
+          else()
+            set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic-errors")
+          endif()
         endif()
     endif()
 endmacro()
@@ -119,29 +131,57 @@ include_directories( ${jsoncpp_SOURCE_DIR}/include )
 if(MSVC)
     # Only enabled in debug because some old versions of VS STL generate
     # unreachable code warning when compiled in release configuration.
-    add_compile_options($<$<CONFIG:Debug>:/W4>)
+    if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
+      add_compile_options($<$<CONFIG:Debug>:/W4>)
+    else()
+      set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} /W4 ")
+    endif()
 endif()
 
 if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     # using regular Clang or AppleClang
-    add_compile_options(-Wall -Wconversion -Wshadow -Werror=conversion -Werror=sign-compare)
+    if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
+      add_compile_options(-Wall -Wconversion -Wshadow -Werror=conversion -Werror=sign-compare)
+    else()
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wconversion -Wshadow -Werror=conversion -Werror=sign-compare")
+    endif()
 elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     # using GCC
-    add_compile_options(-Wall -Wconversion -Wshadow -Wextra)
+    if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
+      add_compile_options(-Wall -Wconversion -Wshadow -Wextra)
+    else()
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wconversion -Wshadow -Wextra")
+    endif()
     # not yet ready for -Wsign-conversion
 
     if(JSONCPP_WITH_STRICT_ISO)
+      if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
         add_compile_options(-pedantic)
+      else()
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic")
+      endif()
     endif()
     if(JSONCPP_WITH_WARNING_AS_ERROR)
+      if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
         add_compile_options(-Werror=conversion)
+      else()
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=conversion")
+      endif()
     endif()
 elseif(CMAKE_CXX_COMPILER_ID STREQUAL "Intel")
     # using Intel compiler
-    add_compile_options(-Wall -Wconversion -Wshadow -Wextra -Werror=conversion)
+    if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
+      add_compile_options(-Wall -Wconversion -Wshadow -Wextra -Werror=conversion)
+    else()
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wconversion -Wshadow -Wextra -Werror=conversion")
+    endif()
 
     if(JSONCPP_WITH_STRICT_ISO AND NOT JSONCPP_WITH_WARNING_AS_ERROR)
+      if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
         add_compile_options(-pedantic)
+      else()
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic")
+      endif()
     endif()
 endif()
 

Diff for: src/jsontestrunner/CMakeLists.txt

@@ -5,7 +5,11 @@ add_executable(jsontestrunner_exe
                )
 
 if(BUILD_SHARED_LIBS)
+  if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
     add_compile_definitions( JSON_DLL )
+  else()
+    add_definitions( -DJSON_DLL )
+  endif()
 endif()
 target_link_libraries(jsontestrunner_exe jsoncpp_lib)
 

Diff for: src/lib_json/CMakeLists.txt

@@ -34,7 +34,11 @@ endif()
 
 if(NOT (HAVE_CLOCALE AND HAVE_LCONV_SIZE AND HAVE_DECIMAL_POINT AND HAVE_LOCALECONV))
     message(WARNING "Locale functionality is not supported")
-    add_compile_definitions(JSONCPP_NO_LOCALE_SUPPORT)
+    if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
+      add_compile_definitions(JSONCPP_NO_LOCALE_SUPPORT)
+    else()
+      add_definitions(-DJSONCPP_NO_LOCALE_SUPPORT)
+    endif()
 endif()
 
 set( JSONCPP_INCLUDE_DIR ../../include )
@@ -59,15 +63,22 @@ set(jsoncpp_sources
                 json_value.cpp
                 json_writer.cpp)
 
+
+              XXX
 # Install instructions for this target
 if(JSONCPP_WITH_CMAKE_PACKAGE)
     set(INSTALL_EXPORT EXPORT jsoncpp)
 else(JSONCPP_WITH_CMAKE_PACKAGE)
     set(INSTALL_EXPORT)
 endif()
 
+
 if(BUILD_SHARED_LIBS)
-    add_compile_definitions( JSON_DLL_BUILD )
+    if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
+      add_compile_definitions( JSON_DLL_BUILD )
+    else()
+      add_definitions( -DJSON_DLL_BUILD )
+    endif()
 endif()
 
 

Diff for: src/test_lib_json/CMakeLists.txt

@@ -10,7 +10,11 @@ add_executable( jsoncpp_test
 
 
 if(BUILD_SHARED_LIBS)
+  if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
     add_compile_definitions( JSON_DLL )
+  else()
+    add_definitions( -DJSON_DLL )
+  endif()
 endif()
 target_link_libraries(jsoncpp_test jsoncpp_lib)