Issue #508: -fPIC to link libjson-c.a with libs

json-c has symbol collisions with other popular C JSON libraries.
To prevent random crashes in downstream applications that may use a
library which depends on json-c, the solution is to statically link
libjson-c.a into those libraries.

`-fPIC`/`-fPIE` is required when building a `.a` so it can be linked
into a `.so`.

CMakeLists.txt

@@ -110,6 +110,14 @@ if (MSVC)
     list(APPEND CMAKE_REQUIRED_FLAGS /wd4996)
 endif()
 
+if (NOT DISABLE_STATIC_FPIC)
+    # Use '-fPIC'/'-fPIE' option.
+    # This will allow other libraries to statically link in libjson-c.a
+    # which in turn prevents crashes in downstream apps that may use
+    # a different JSON library with identical symbol names.
+    set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+endif()
+
 check_include_file("fcntl.h"        HAVE_FCNTL_H)
 check_include_file("inttypes.h"     HAVE_INTTYPES_H)
 check_include_file(stdarg.h         HAVE_STDARG_H)

README.md

@@ -99,6 +99,7 @@ CMAKE_INSTALL_PREFIX         | String | The install location.
 CMAKE_BUILD_TYPE             | String | Defaults to "debug".
 BUILD_SHARED_LIBS            | Bool   | The default build generates a dynamic (dll/so) library.  Set this to OFF to create a static library only.
 BUILD_STATIC_LIBS            | Bool   | The default build generates a static (lib/a) library.  Set this to OFF to create a shared library only.
+DISABLE_STATIC_FPIC          | Bool   | The default builds position independent code.  Set this to OFF to create a shared library only.
 DISABLE_BSYMBOLIC            | Bool   | Disable use of -Bsymbolic-functions.
 DISABLE_THREAD_LOCAL_STORAGE | Bool   | Disable use of Thread-Local Storage (HAVE___THREAD).
 DISABLE_WERROR               | Bool   | Disable use of -Werror.