[a/b]

Try this (if the compiler doesn't support concat when macro parameters are expanded at the same time):

#ifdef CURL_ISOCPP
#define CFINIT2(x,y) x ## y
#define CFINIT(name) CFINIT2(CURLFORM_,name)
#else
/* The macro "##" is ISO C, we assume pre-ISO C doesn't support it. */
#define CFINIT2(x,y) x/**/y
#define CFINIT(name) CFINIT2(CURLFORM_,name)
#endif

or remove macros altogether:

typedef enum {
  CURLFORM_NOTHING,
  CURLFORM_COPYNAME,
...
  CURLFORM_CONTENTLEN,
  CURLFORM_LASTENTRY
} CURLformoption;