Obsolete C libraries may not provide isnan/isfinite macros

[jphickey]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I performed a cursory search to see if the bug report is relevant, not redundant, nor in conflict with other tickets.

Describe the bug
Although the isnan and isfinite macros have been specified by ISO C for over 2 decades, there are still some (obsolete) C libraries in use that do not provide these.

To Reproduce
Compile LC on VxWorks 6.9, error is reported:

apps/lc/fsw/src/lc_watch.c: In function 'LC_FloatCompare':
apps/lc/fsw/src/lc_watch.c:680: error: implicit declaration of function 'isnan'
apps/lc/fsw/src/lc_watch.c: In function 'LC_ValidateWDT':
apps/lc/fsw/src/lc_watch.c:1016: error: implicit declaration of function 'isfinite'

Expected behavior
Should build successfully

System observed on:
VxWorks 6.9, GNU C compiler version 4.3

Additional context
This appears to be a bug in the math.h header. It has these macros defined, but they are conditional on GNUC version <= 2. As this is using GNU C version 4.3, it does not provide them. But nothing else does, either.

Reporter Info
Joseph Hickey, Vantage Systems, Inc.