package/libfastjson: fix build without threads

package/libfastjson: fix build without threads

libfastjson is supposed to use the __sync_*4 atomic intrinsics, but alas
it is not using them because their ./configure decides they are not
available: it uses AC_TRY_RUN to check for them, and the default is to
decide they are not available, because of cross-compilation.

Besides, one of the source files was not including the generated
config.h, so even after fixing ./configure there was still a build
error.

The first patch is a backport from upstream to fix the missing
inclusion.

The second patch is switching AC_TRY_RUN over to AC_LINK_IFELSE, as the
only thing we're interested in is to check for the presence of the
atomic intrisics, and linking is enough for that.

Fixes:
    http://autobuild.buildroot.org/results/192/1923d0b570adba494f83747a9610ea6ec35f5223/
    http://autobuild.buildroot.org/results/23a/23ac0e742ed3a70ae4d038f8c9eadc23e708f671/
    and many others...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>