A somewhat condensed, C99 version:

#include <stdio.h>
#include <ctype.h>
#include <string.h>

char *str_rwords(char *s)
{
    int n = strlen(s);
    char t[n], *e = t + n, *d = e, *s0;
    do {
        for (s0 = s; *s && isalpha(*s); s++);
        if ((n = s - s0)) {
            if (d != e) *--d = ' ';
            d = memcpy(d - n, s0, n);
        }
    } while (*s++);
    return strndup(d, e - d);
}

int main(void)
{
    puts(str_rwords("   one   two \t three four  "));
}
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