libjson-c: update to 0.15

Drop patches as they've been upstreamed:
 * 001-Fix-CVE-2020-12762.patch

Refresh patches:
 * 000-libm.patch

Add patch to avoid build failure due to missing docs in tarball.

Signed-off-by: David Bauer <mail@david-bauer.net>

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

@@ -1,6 +1,6 @@
 --- a/math_compat.h
 +++ b/math_compat.h
-@@ -6,31 +6,9 @@
+@@ -6,38 +6,9 @@
   * @brief Do not use, json-c internal, may be changed or removed at any time.
   */
  
@@ -10,6 +10,9 @@
 -#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
 -
@@ -17,6 +20,10 @@
 -#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
 -

package/libs/libjson-c/patches/001-dont-build-docs.patch

@@ -0,0 +1,11 @@
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -401,8 +401,6 @@ set(JSON_C_SOURCES
+ include_directories(${PROJECT_SOURCE_DIR})
+ include_directories(${PROJECT_BINARY_DIR})
+ 
+-add_subdirectory(doc)
+-
+ # uninstall
+ add_custom_target(uninstall
+   COMMAND cat ${PROJECT_BINARY_DIR}/install_manifest.txt | xargs rm