libjson-c: remove old math patch

Remove old math patch meant for old GCC versions. It's not needed
for GCC and causes issues with clang.

Add CMake patch to identify clang properly and apply the proper
flags. Fixes the following warnings/errors:

json_pointer.c:230:7: warning: implicit declaration of function
'vasprintf' is invalid in C99 [-Wimplicit-function-declaration]
        rc = vasprintf(&path_copy, path_fmt, args);
             ^
json_pointer.c:317:7: warning: implicit declaration of function
'vasprintf' is invalid in C99 [-Wimplicit-function-declaration]
        rc = vasprintf(&path_copy, path_fmt, args);
             ^
/usr/include/bits/mathcalls.h:177:23: error: cannot redeclare builtin
function '__builtin_isinf'
__MATHDECL_ALIAS (int,isinf,, (_Mdouble_ __value), isinf)
                      ^
/usr/include/bits/mathcalls.h:177:23: note: '__builtin_isinf' is a
builtin with type 'int ()'
/usr/include/bits/mathcalls.h:213:23: error: cannot redeclare builtin
function '__builtin_isnan'
__MATHDECL_ALIAS (int,isnan,, (_Mdouble_ __value), isnan)

The clang patch is an upstream backport.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

package/libs/libjson-c/patches/000-libm.patch

@@ -1,45 +0,0 @@
---- a/math_compat.h
-+++ b/math_compat.h
-@@ -6,38 +6,9 @@
-  * @brief Do not use, json-c internal, may be changed or removed at any time.
-  */
- 
--/* Define isnan, isinf, infinity and nan on Windows/MSVC */
--
--#ifndef HAVE_DECL_ISNAN
--#ifdef HAVE_DECL__ISNAN
--#include <float.h>
--#define isnan(x) _isnan(x)
--#else
--/* On platforms like AIX and "IBM i" we need to provide our own isnan */
--#define isnan(x) ((x) != (x))
--#endif
--#endif
--
--#ifndef HAVE_DECL_ISINF
--#ifdef HAVE_DECL__FINITE
--#include <float.h>
--#define isinf(x) (!_finite(x))
--#else
--#include <float.h>
--/* On platforms like AIX and "IBM i" we need to provide our own isinf */
--#define isinf(x) ((x) < -DBL_MAX || (x) > DBL_MAX)
--#endif
--#endif
--
--#ifndef HAVE_DECL_INFINITY
--#include <float.h>
--#define INFINITY (DBL_MAX + DBL_MAX)
--#define HAVE_DECL_INFINITY
--#endif
--
--#ifndef HAVE_DECL_NAN
--#define NAN (INFINITY - INFINITY)
--#define HAVE_DECL_NAN
--#endif
-+#undef isnan
-+#define isnan(x) __builtin_isnan(x)
-+#undef isinf
-+#define isinf(x) __builtin_isinf(x)
- 
- #endif

package/libs/libjson-c/patches/010-clang.patch

@@ -0,0 +1,11 @@
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -265,7 +265,7 @@ message(STATUS "Wrote ${PROJECT_BINARY_D
+ configure_file(${PROJECT_SOURCE_DIR}/cmake/json_config.h.in   ${PROJECT_BINARY_DIR}/json_config.h)
+ message(STATUS "Wrote ${PROJECT_BINARY_DIR}/json_config.h")
+ 
+-if ("${CMAKE_C_COMPILER_ID}" STREQUAL "GNU")
++if ("${CMAKE_C_COMPILER_ID}" STREQUAL "GNU" OR "${CMAKE_C_COMPILER_ID}" STREQUAL "Clang")
+     set(CMAKE_C_FLAGS           "${CMAKE_C_FLAGS} -ffunction-sections -fdata-sections")
+ 	if ("${DISABLE_WERROR}" STREQUAL "OFF")
+ 	    set(CMAKE_C_FLAGS           "${CMAKE_C_FLAGS} -Werror")