cmake: remove lib prefix when building for non windows hosts

Add inline functions for readFile and writeFile for std::string

Fix portability problem due to == test

Add --disable-doc option

Improve the treatment of single line comment.

The "lib"-prefix is appended automatically for most non windows hosts.
Using the library name "libconfig" results in an .so file named
liblibconfig.so.

Instead, we use just "config" when the building for non windows hosts.

The "test" command, as well as the "[" command, are not required to know
the "==" operator. Only a few implementations like bash and some
versions of ksh support it.

When you run "test foo == foo" on a platform that does not support the
"==" operator, the result will be "false" instead of "true". This can
lead to unexpected behavior.

Make including library easier

libconfig can now be built by including with eg. add_subdirectory
directly, without changing compiler flags

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

Some changes for CMakeLists.txt - one bug-fix included

Setting this define makes that

    #include <stdlib.h>

does nothing (because of the include-guard). But this file
is needed for free() and realloc() and others.

The cmake built-in variable BUILD_SHARED_LIBS
does exactly what LIB_TYPE was used for. Switching between
SHARED and STATIC library-builds.

Fixed an issue causing build to fail on OSX 10.12.

#96 : fix lib/CMakeLists.txt, add wincompat.c.

Added optional method chained interface

AM_CONFIG_HEADER is deprecated

configure cmake

Safely handle escape sequences