view acinclude.m4 @ 1127:9fb8dfd7ce4c spectrogram-minor-refactor

Fix threshold in spectrogram -- it wasn't working in the last release.
There is a new protocol for this. Formerly the threshold parameter had a
range from -50dB to 0 with the default at -50, and -50 treated internally
as "no threshold". However, there was a hardcoded, hidden internal threshold
for spectrogram colour mapping at -80dB with anything below this being rounded
to zero. Now the threshold parameter has range -81 to -1 with the default
at -80, -81 is treated internally as "no threshold", and there is no hidden
internal threshold. So the default behaviour is the same as before, an
effective -80dB threshold, but it is now possible to change this in both
directions. Sessions reloaded from prior versions may look slightly different
because, if the session says there should be no threshold, there will now
actually be no threshold instead of having the hidden internal one.

Still need to do something in the UI to make it apparent that the -81dB
setting removes the threshold entirely. This is at least no worse than the
previous, also obscured, magic -50dB setting.

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# From autoconf archive:

# ============================================================================
#  http://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_11.html
# ============================================================================
#
# SYNOPSIS
#
#   AX_CXX_COMPILE_STDCXX_11([ext|noext],[mandatory|optional])
#
# DESCRIPTION
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#   Check for baseline language coverage in the compiler for the C++11
#   standard; if necessary, add switches to CXXFLAGS to enable support.
#
#   The first argument, if specified, indicates whether you insist on an
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#   should error out if no mode with that support is found.  If specified
#   'optional', then configuration proceeds regardless, after defining
#   HAVE_CXX11 if and only if a supporting mode is found.
#
# LICENSE
#
#   Copyright (c) 2008 Benjamin Kosnik <[e-mail]>
#   Copyright (c) 2012 Zack Weinberg <[e-mail]>
#   Copyright (c) 2013 Roy Stogner <[e-mail]>
#   Copyright (c) 2014 Alexey Sokolov <[e-mail]>
#
#   Copying and distribution of this file, with or without modification, are
#   permitted in any medium without royalty provided the copyright notice
#   and this notice are preserved. This file is offered as-is, without any
#   warranty.

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