cgraph: [nfc] guard 'strcasecmp' definition to avoid duplication

cgraph: [nfc] guard 'strcasecmp' definition to avoid duplication

When compiling with Microsoft Visual Studio 2019, the compiler emits:

  …\lib\cgraph\strcasecmp.h(12,64): warning C4211: nonstandard extension used:
    redefined extern to static

Investigation reveals libgd defines their own `strcasecmp` shim in gd.h:

  /* VS2012+ disable keyword macroizing unless _ALLOW_KEYWORD_MACROS is set
     We define inline, and strcasecmp if they're missing
  */
  #ifdef _MSC_VER
  #  define _ALLOW_KEYWORD_MACROS
  #  ifndef inline
  #    define inline __inline
  #  endif
  #  ifndef strcasecmp
  #    define strcasecmp _stricmp
  #  endif
  #endif

To avoid this warning, guard Graphviz’ shim so it is only used when the libgd
shim is not in use.

Note that this also explains why MSVC is able to compile diffimg.c despite the
Graphviz shim file not being included.