otherwise, building the regex library fails on missing config.h:

    gcc -Iregex-0.12 -I/usr/include/pcap -Os -fomit-frame-pointer -g -DSTDC_HEADERS -I. -DHAVE_CONFIG_H -DLINUX  -D_BSD_SOURCE=1 -D__FAVOR_BSD=1 -Os -fomit-frame-pointer  -c -o regex.o regex.c
    regex.c:43:10: fatal error: config.h: No such file or directory
       43 | #include "config.h"
          |          ^~~~~~~~~~
    compilation terminated.



--- a/Makefile.in
+++ b/Makefile.in
@@ -6,7 +6,7 @@
 CC=@CC@
 
 CPPFLAGS = @DEFS@ -D@OS@ @EXTRA_DEFINES@ @CPPFLAGS@
-CFLAGS   = @EXTRA_INCLUDES@ @CFLAGS@ 
+CFLAGS   = @EXTRA_INCLUDES@ @CFLAGS@ -I@abs_srcdir@
 LDFLAGS  = @EXTRA_LDFLAGS@ @LDFLAGS@
 LIBS     = @LIBS@ @EXTRA_LIBS@