In certain C libraries (e.g uClibc), isnan() and related functions are
implemented in libm, so json-c needs to link against it. This commit
therefore adds an AC_TRY_LINK() test to check whether a program
calling isnan() can be properly linked with no special flags. If not,
we assume linking against libm is needed.

The json-c.pc.in file is also adjusted so that in the case of static
linking against json-c, -lm is also used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Changing root name of library to json-c, and also the
directory where header files are installed to .../jsdon-c/*.

This avoids clashes with other implementations of JSON libraries.