[PATCH,Fortran] Handle 'q' exponent-letter in real-literal-constant

[Steve Kargl <sgk at troutmask dot apl dot washington dot edu>]

Historically, gfortran has accepted real-literal-constants
of the form 1.23Q45 as single precision values.  Many commercial
compilers (dating back years) have used the 'Q' exponent-letter
to mean quadruple precision.  With the addition of software
support for REAL(16) on i386 and x86_64 targets, I anticipate
an increase in use of the 'Q' form.  The attached patch does
the following:

1) If REAL(16) is available, a real-literal-constant with a 'Q'
   exponent-letter is accepted as a REAL(16) entity.

2) If REAL(16) is not available but REAL(10) is, then the constant
   is accepted as a REAL(10).

3) If neither REAL(16) nor REAL(10) is available, an error is 
   issued.

4) An error is issued if one uses -std=f95, f2003, or f2008; otherwise
   a warning will be issued.  The only way to disable the warning is
   to either fix the code to conform to the Fortran standard or use
   -w to disable warnings.

5) If the constant has the form 1.23Q45_16 (ie., a integer kind
   suffix is appended to the value), then an error is issued.  This
   is similar to the requirement that 1.23D45_xyz violated the 
   Fortran Standard.

The attached patch has been built and regression
tested on x86_64-*-freebsd.  There were no regression
with the patch.  OK for trunk (and 4.6 branch after
testing)?

2011-04-25  Steven G. Kargl  <kargl@gcc.gnu.org.>

	PR fortran/48720
	* gfortran.texi: Document the 'Q' exponent-letter extension.
	* primary.c (match_real_constant):  Accept 'Q' as exponent-letter
	for REAL(16) real-literal-constant with a fallback to REAL(10) or
	error if REAL(10) is not available.

-- 
Steve