clean up Makefile for c api
clean up Makefile for c api

- remove assignment of LOCALEDIR since it is assigned in configure.ac.
- remove use of localedir since it is not needed by the API.
- define the header to install correctly using include_HEADERS.
- separate AM_CFLAGS into libspeechd_la_CFLAGS and libspeechd_la_CPPFLAGS
- convert autoconf substitutions to make variables
- move thread library reference to libspeechd_la_libadd line

Reviewed-by: Andrei Kholodnyi <8ed355542eeb21dc16259915650192a25bd4bf1a@gmail.com>

use INCLUDES instead of inc_local

Modified-by: Andrei Kholodnyi <8ed355542eeb21dc16259915650192a25bd4bf1a@gmail.com>

razr/speechd,razr/speechd,razr/speechd,razr/speechd