Fix link with jansson

[orgads]

jansson/lib/libjansson.a(value.o): In function `json_real':
value.c:(.text+0x3134): undefined reference to `__isnan'
value.c:(.text+0x314c): undefined reference to `__isinf'
jansson/lib/libjansson.a(value.o): In function `json_real_set':
value.c:(.text+0x3294): undefined reference to `__isnan'
value.c:(.text+0x32ac): undefined reference to `__isinf'
collect2: error: ld returned 1 exit status

[JoshMcCullough]

@fitzyjoe can you review this, please?

[JoshMcCullough]

@orgads thanks for the PR! Can you explain what -lm does for us here?

[orgads]

Links with the standard math lib. The functions _isnan and _isinf are used in jansson.

[JoshMcCullough]

This looks good, just needs to be rebased.

[orgads]

Thank you.