Add pragma pack directive

[sergiy80]

Added pragma pack directive to jsoncpp headers.
Related to #458

[sergiy80]

To reproduce the issue.
In Visual Studio project (makefiles/msvc2010) just change test_lib_json's Struct Memeber Alignment option from Default (8) to 2, leaving lib_json's option as is (Default).
Without this commit test_lib_json.exe will produce either failed tests or abnormal termination.

[cdunn2001]

Was this a problem anywhere other than Windows/VisualStudio?

[sergiy80]

Also I could reproduce seg fault error compiling jsoncpp using gcc and clang under MSYS2 environment.
Beause linux standard system headers do not have packing guards (unlike VS standard headers), it is not possible just to use packing option other then default (8) for whole project.
To reproduce the crash need to modify jsontest.cpp and main.cpp in test_lib_json project to exclude standard headers from changing packing opton.
The diffs are below.
With these changes in the test project the results are the same as for VS project, failed tests and segmentation fault error.

diff --git a/src/test_lib_json/jsontest.cpp b/src/test_lib_json/jsontest.cpp
index 4c10a37..08e193f 100644
--- a/src/test_lib_json/jsontest.cpp
+++ b/src/test_lib_json/jsontest.cpp
@@ -68,6 +68,8 @@
 #include <windows.h>
 #endif // if defined(_WIN32)
 
+#pragma pack(2)
+
 namespace JsonTest {
 
 // class TestResult

diff --git a/src/test_lib_json/main.cpp b/src/test_lib_json/main.cpp
index 47cd981..623a692 100644
--- a/src/test_lib_json/main.cpp
+++ b/src/test_lib_json/main.cpp
@@ -3,15 +3,19 @@
 // recognized in your jurisdiction.
 // See file LICENSE for detail or copy at http://jsoncpp.sourceforge.net/LICENSE
 
-#include "jsontest.h"
-#include <json/config.h>
-#include <json/json.h>
+
 #include <cstring>
 #include <limits>
 #include <sstream>
 #include <string>
 #include <iomanip>
 
+#pragma pack(2)
+#include "jsontest.h"
+#include <json/config.h>
+#include <json/json.h>
+
 // Make numeric limits more convenient to talk about.
 // Assumes int type in 32 bits.
 #define kint32max Json::Value::maxInt

[cdunn2001]

Thanks. Very interesting. I did not know about this pragma.