Link against libm when needed

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>

json-c.pc.in

@@ -6,6 +6,7 @@ includedir=@includedir@
 Name: json-c
 Description: JSON implementation in C
 Version: @VERSION@
-Requires: 
+Requires:
+Libs.private: @LIBS@
 Libs:  -L${libdir} -ljson-c
 Cflags: -I${includedir}/json-c