Remove EXT_LIB_PATH variable.

Fix a copy-and-paste leftover in the check for libcintelhex.  The
variable was set but never used.
Save and restore CPPFLAGS and LDFLAGS.

When testing for the libcintelhex installation path, the flags
variables should stay unmodified if the respective tests fail.
Introduce saved_* variables to hold the original values.
Fix uninitialized local variable.

The count variable in symbol_list_foreach_count() was optimized out by
GCC with -O2, leading to a constant (undefined) return value of zero.
Initialize the count to zero so that the increment is not optimized
away.
List header files in *_SOURCES.

To be included in the release distribution, header files must also be
listed as program sources.  Use the order defined in include_order.txt
and list the latter in EXTRA_DIST to be included in a source release.
Fix configure check for internal libcintelhex header.

When building from a separate build tree (VPATH build), the header
check for the internal embedded libcintelhex copy must look relative
to the source tree, so add $(srcdir) to the include search path
option.

The logic in src/Makefile.am takes care of setting the needed CPPFLAGS
and LDFLAGS conditionally, so just restore the old CPPFLAGS value and
leave LDFLAGS untouched in configure.ac.
Update to newer libcintelhex release.

The build system for libcintelhex has been dramatically improved and
is now using GNU automake and GNU libtool.  This makes it a lot easier
to build this package with an embedded copy, including support for
VPATH builds.

Adjust the library location for the new libtool path.  The elf-mangle
package does not use libtool yet.