[plot2fig] Use the POSIX regex functions on Linux instead of the old/obsolete BSD ones.

[vkalintiris]

Summary

Use regcmp() and regex() instead of re_comp() and re_exec(). This fixes
the build failure when using the Musl C library.

MultiSource/Benchmarks/Prolangs-C/plot2fig/fontname.c

for (i=0; i<len2; i++)
     return 1;
   }
     }
   return 0;
 }
 
 int font_id = 0; /* the default font */
 
-#ifdef SOLARIS
+#if defined(SOLARIS) || defined(__linux__)
 /* XZ: use regcmp() and regex() instead of re_comp() and re_exec() */
 /* PAS (6/3/96): use regcomp() and regexec() from regex.h instead of
   re_comp() and re_exec() on the understanding that it only has to
   compile for our purposes, not run.  */
 
 #include <regex.h>
 
 int fontname(char *s)