Split "gztool.c" for usage as library

Splits "gztool.c" into components for use in external libraries.

1. gztool.h: Public API including structs and definitions that end-users of the
library would need to us. Modifications to API cause major version number bump.

2. gztool.c: Implementation of the public functions that IDEALLY users of
'gztool.h' would not need to read

3. gztool_internal.h:  Internal API, which can change between versions

4. gztool_internal.c: Implementation of the internal API

5. main.c: Existing CLI application

Unfortunately some key functions (relating to creating index, decompressing and
compressing) to the public API are far from ideal to be exposed in a public API
(especially around the modal enum that modifies function behavior), but this
minor change was concurred as acceptable by the author of gztool [1].

[1] #23 (comment)

Author: shasheene

Makefile.am

@@ -4,7 +4,7 @@ TESTS_ENVIRONMENT = \
         gztool_abspath='$(abs_top_builddir)'/gztool; \
 	export gztool_abspath;
 bin_PROGRAMS = gztool
-gztool_SOURCES = gztool.c
+gztool_SOURCES = gztool.c gztool_internal.c main.c
 gztool_LDADD = -lm -lz
 dist_doc_DATA = README.md
 dist_man_MANS = gztool.1

README.md

@@ -71,7 +71,7 @@ Compilation
 
     $ sudo apt-get install zlib1g-dev
 
-    $ gcc -O3 -o gztool gztool.c -lz -lm
+    $ gcc -O3 -o gztool gztool.c gztool.h gztool_internal.c gztool_internal.h main.c -lz -lm
 
 If you wish you can use autoconf to check the dependencies, build and
 test `gztool`: