configure: Use pkg-config for jimtcl

The jimtcl project supports pkg-config, use it for a simpler
configuration of compiler and linker flags and to enforce the minimum
required package version.

Since the jimtcl pkg-config file is not available on all systems, use
AC_CHECK_HEADER() as fallback.

Change-Id: I6fdcc818a8fdd205a126b0a46356434dbe890226
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/8383
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

Makefile.am

@@ -48,6 +48,8 @@ AM_CPPFLAGS = $(HOST_CPPFLAGS)\
 if INTERNAL_JIMTCL
 AM_CPPFLAGS += -I$(top_srcdir)/jimtcl \
 			   -I$(top_builddir)/jimtcl
+else
+AM_CPPFLAGS += $(JIMTCL_CFLAGS)
 endif
 EXTRA_DIST += \
 	BUGS \