b0rken Solaris-doesn't-have-setenv fix

[Josh Sled]

src/core-utils/core-utils.h uses the config.h HAVE_SETENV
to #define 'gnc_(un)setenv' to the system (un)setenv ... but
src/core-utils/core-utils.c defines them anyways.

If you HAVE_SETENV, the functions in core-utils.c get preprocessed from:
int gnc_unsetenv( ... ) { /* body */ }
into:
int unsetenv( ... ) { /* body */ }
which results in the following compile-time error:

core-utils.c:63: conflicting types for `unsetenv'
/usr/include/stdlib.h:550: previous declaration of `unsetenv'

as my unsetenv is declared:
void unsetenv(const char *name);

...jsled