graphics/sane-backends: fix build with clang 15

During an exp-run for llvm 15 (see bug 265425), it turned out that
graphics/sane-backends failed to build with clang 15:

  jpegtopdf.c:204:5: error: incompatible pointer to integer conversion assigning to 'long' from 'char *(int, int)' [-Wint-conversion]
          tz = timezone;
             ^ ~~~~~~~~
  scanimage.c:1248:42: warning: call to undeclared function 'basename'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
                png_set_iCCP(*png_ptr, *info_ptr, basename(icc_profile), PNG_COMPRESSION_TYPE_BASE, icc_buffer, icc_size);
                                                  ^
  scanimage.c:1248:42: error: incompatible integer to pointer conversion passing 'int' to parameter of type 'png_const_charp' (aka 'const char *') [-Wint-conversion]
                png_set_iCCP(*png_ptr, *info_ptr, basename(icc_profile), PNG_COMPRESSION_TYPE_BASE, icc_buffer, icc_size);
                                                ^~~~~~~~~~~~~~~~~~~~~

The jpegtopdf error is because 'timezone' as a global external variable
does not exist on FreeBSD: it is a glibc-ism. Use struct tm's tm_gmtoff
field instead, which also has its sign reversed from the glibc global.

The scanimage error is because basename(3) is used without including
<libgen.h>. With glibc, basename() is declared in both string.h and
libgen.h, where the former gives you the GNU specific variant, and the
latter the POSIX variant.

PR:		268771
Approved by:	vvd@unislabs.com (maintainer)
MFH:		2023Q1

graphics/sane-backends/files/patch-frontend_jpegtopdf.c

@@ -0,0 +1,14 @@
+--- frontend/jpegtopdf.c.orig	2022-01-19 16:38:00 UTC
++++ frontend/jpegtopdf.c
+@@ -201,7 +201,11 @@ static SANE_Int _get_current_time( struct tm *pt, SANE
+ 		goto EXIT;
+ 	}
+ 	/* get time difference ( OHH'mm' ) */
++#ifdef __FreeBSD__
++	tz = -pt->tm_gmtoff;
++#else
+ 	tz = timezone;
++#endif
+ 	if ( tz > 0 ) {
+ 		*sign_c = '-';
+ 	}

graphics/sane-backends/files/patch-frontend_scanimage.c

@@ -0,0 +1,12 @@
+--- frontend/scanimage.c.orig	2022-01-19 16:38:00 UTC
++++ frontend/scanimage.c
+@@ -39,6 +39,9 @@
+ #include <string.h>
+ #include <unistd.h>
+ #include <stdarg.h>
++#ifdef __FreeBSD__
++#include <libgen.h>
++#endif
+ 
+ #include <sys/types.h>
+ #include <sys/stat.h>