Added ladislav suggestion of using  David M. Gay's dtoa for molding decimals

Currently, R3 uses platform-specific code for molding decimal numbers.
This increases the effort necessary to port to new platforms like
android-arm or linux-arm

Porting R3 to android-arm we found none of the system-specific methods
currently used by R3 were available. Instead of adding another
system-specific method specifically for android-arm, we decided to
replace the system-specific methods using the dtoa function
implementation by David M. Gay.

David M. Gay's original code is available at:
http://www.netlib.org/fp/dtoa.c

This implementation brings the following advantages:

MOLD and MOLD/ALL results will be accurate (solving cc#897).
MOLD results will be consistent across wide range of platforms (solving
cc#1633).
MOLD results will be optimal: yielding the shortest string that can be
used to reproduce the number (i.e. a string that is accurate enough).
Makes porting to new platforms easier.
Battle-tested solution, as this is the way how the problem was solved in
e.g. Java, Javascript, Python, Firefox, Chrome.

The commit makes the following changes:

David M. Gay's implementation is added as src/core/f-dtoa.c. The source
is unmodified except for an #include of Rebol-specific configuration and
a reference to where the source was downloaded from.
An attribution notice is added to NOTICE.
Configuration for f-dtoa.c is contained in src/include/reb-dtoa.h.
The additional source is listed in src/tools/file-base.r.
make/makefile is re-generated to accommodate the additional source.
The HAS_ECVT, HAS_LOG10L, HAS_LONG_DOUBLE Rebol configuration defines
are removed as they are no longer necessary.
HAS_LL_CONSTS was previously implied by HAS_LONG_DOUBLE; it is now
defined explicitly for the respective platforms.
Functions for decimal conversion are modified to use dtoa.

This resolves CureCode issues 897, 1633, 1942.