Fix typo on Makefile warning messages.
Print warning instead of echo message if playrec will be disabled.

* src/configure.ac: print warning instead of echo message about disabling
playrec. This is specially important for Octave because by default it won't
display make's STDOUT, only STDERR, leaving the users without notice that
this feature was disabled.
Always build playrec.mex but give an error if disabled.

* instead of completely disablingg the build of playrec when playaudio is
not available, use pre-processor to have the function give an error about
being disabled at build-time.  The effect of this is that the function will
always be present, and give a useful error message to the user reminding
him why it failed and how to fix (rebuild with available).
fix in signals requirements
Safer test_demos

demos are now run in a separate function so that valiables are not
overwritten.
Updated .gitignore
Merge remote-tracking branch 'forge/makefile-typo'

Typos in makefiles fixed by Carne Draug.
Fixed handling of non-additive costs in wpbest

and some minor changes.
Merging changes to the playrec compilation
Making the build system to work with the HAVE_PORTAUDIO flag
block_interface is now C89 compilant
Muting some compilation warnings
Added help file for playrec