Fix fails to build with -fno-common or gcc-10

See also: https://bugs.gentoo.org/706366

gcc-10 and above flipped a default from -fcommon to -fno-common:
https://gcc.gnu.org/PR85678

Usually all it takes is to add a few 'extern' declarations and
move definitions from header files to modules. I've port iverilog
to gcc-10 accroding to this guide:
https://wiki.gentoo.org/wiki/Gcc_10_porting_notes/fno_common

To fix this, I analyzed the code, and found ``pli_trace`` has been
defined at here:
https://github.com/steveicarus/iverilog/blob/v10_3/libveriuser/priv.c#L24

So I changed ``FILE* pli_trace;`` to ``extern FILE* pli_trace;``.

The var ``current_file`` only in ``cfparse_misc.h``, I changed it
from ``char *current_file;`` to ``extern char *current_file;`` and
declaring it in cflexor.lex

And then it works.

Signed-off-by: Huang Rui <vowstar@gmail.com>
(cherry picked from commit d49d26a5c5)

driver/cflexor.lex

@@ -27,6 +27,8 @@
 # include  "globals.h"
 # include  <string.h>
 
+char *current_file = NULL;
+
 static int comment_enter;
 static char* trim_trailing_white(char*txt, int trim);
 

driver/cfparse_misc.h

@@ -39,6 +39,6 @@ int cferror(const char *);
 int cfparse(void);
 void switch_to_command_file(const char *);
 void destroy_lexor(void);
-char *current_file;
+extern char *current_file;
 
 #endif /* IVL_cfparse_misc_H */

libveriuser/priv.h

@@ -31,6 +31,6 @@ extern char* __acc_newstring(const char*txt);
 /*
  * Trace file for logging ACC and TF calls.
  */
-FILE* pli_trace;
+extern FILE* pli_trace;
 
 #endif /* IVL_priv_H */