[base] Refactor experimental (disabled) BBox_Cubic_Check.

* src/base/ftbbox.c (BBox_Cubic_Check): Implement the minimum search
as the mirror image of the maximum search.
[base] Finish experimental (disabled) BBox_Cubic_Check implementation.

* src/base/ftbbox.c (BBox_Cubic_Check): Scale arguments to improve
accuracy and avoid overflows.
[base] Clean up BBox_Conic_Check.

* src/base/ftbbox.c (BBox_Conic_Check): Remove redundant checks for
extremum at the segment ends, which are already within the bbox.
Slightly modify calculations.
Fix Savannah bug #39804.

* builds/unix/configure.raw (LIBPNG): Define and export.
* builds/unix/freetype-config.in, builds/unix/freetype2.in: Handle
libpng.
* builds/unix/unix-def.in (freetype2.pc): Don't set executable bit.
[base] Enable new algorithm for BBox_Cubic_Check.

* src/base/ftbbox.c: Enable new BBox_Cubic_Check algorithm, remove the
old one. Improve comments.
[autofit] Introduce data file for blue strings.

The idea is to have a central file which gets processed by a Perl
script to create proper `.c' and `.h' files using templates.  There
are two other reasons to do that:

  . The data file should be easily readable.  We use UTF-8 encoding
    which then gets converted to single bytes.

  . Since the number of supported scripts will increase soon, the
    current usage of blue string arrays is a waste of space.  Using
    the Perl script it is possible to imitate jagged arrays,
    defining enumeration constants as offsets into the arrays.

This commit only adds files without changing any functionality.

* src/autofit/afblue.dat: New data file.
* src/tools/afblue.pl: New Perl script for processing `afblue.dat'.

* src/autofit/afblue.cin, src/autofit/afblue.hin: New template files
for...
* src/autofit/afblue.c, src/autofit/afblue.c: New source files.
To avoid a dependency on Perl, we add them too.
Merge branch 'master' of git://git.sv.nongnu.org/freetype/freetype2