Add option JSONCPP_WITH_STRICT_ISO

'-pedantic' issues all warnings demanded by strict ISO C/C++; rejecting
extensions that do not follow ISO C/C++. Without this option, certain GNU
extensions and traditional C/C++ features are supported as well.

With this option enabled building jsoncpp fails with the musl toolchain on
x86 because of an incompatible posix_memalign declaration [1]. Without
'-pedantic' there is no error anymore and jsoncpp builds fine.

Add an option JSONCPP_WITH_STRICT_ISO to disable compilation with '-pedantic'
with GCC. If jsoncpp is build with the JSONCPP_WITH_WARNING_AS_ERROR option
'-pedantic-errors' is used instead.

[1] https://gcc.gnu.org/ml/gcc-patches/2015-05/msg01425.html

CMakeLists.txt

@@ -7,6 +7,7 @@ ENABLE_TESTING()
 OPTION(JSONCPP_WITH_TESTS "Compile and (for jsoncpp_check) run JsonCpp test executables" ON)
 OPTION(JSONCPP_WITH_POST_BUILD_UNITTEST "Automatically run unit-tests as a post build step" ON)
 OPTION(JSONCPP_WITH_WARNING_AS_ERROR "Force compilation to fail if a warning occurs" OFF)
+OPTION(JSONCPP_WITH_STRICT_ISO "Issue all the warnings demanded by strict ISO C and ISO C++" ON)
 OPTION(JSONCPP_WITH_PKGCONFIG_SUPPORT "Generate and install .pc files" ON)
 OPTION(JSONCPP_WITH_CMAKE_PACKAGE "Generate and install cmake package files" OFF)
 OPTION(BUILD_SHARED_LIBS "Build jsoncpp_lib as a shared library." OFF)
@@ -83,6 +84,10 @@ macro(UseCompilationWarningAsError)
         # Only enabled in debug because some old versions of VS STL generate
         # warnings when compiled in release configuration.
 		set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} /WX ")
+  elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    if (JSONCPP_WITH_STRICT_ISO)
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic-errors")
+    endif ()
 	endif()
 endmacro()
 
@@ -100,8 +105,12 @@ if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -Wconversion -Wshadow -Wno-sign-conversion")
 elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
   # using GCC
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -Wconversion -Wshadow -Wextra -pedantic")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -Wconversion -Wshadow -Wextra")
   # not yet ready for -Wsign-conversion
+
+  if (JSONCPP_WITH_STRICT_ISO AND NOT JSONCPP_WITH_WARNING_AS_ERROR)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic")
+  endif ()
 endif()
 
 find_program(CCACHE_FOUND ccache)