variable for alternative extensions

[katjav]

Pd externals for Linux have extension 'pd_linux' by default but Pd also knows a couple of architecture specific Linux extensions: 'l_i386', 'l_ia64' and 'l_arm'. This is useful for 'fat library' distributions (because you can't easily make fat binaries for Linux). For convenience Makefile.pdlibbuilder could build with an appropriate alternative extension if a user specifies a certain variable, like use-alternative-extension=yes.

[katjav]

Now that Pd and externals can be built for 64 bit Windows, the option to distribute 'fat libraries' and the need for distinctive extensions becomes even more relevant. But I'm also thinking of self-contained 'fat Pd projects' with all external dependencies embedded.

Pd doesn't (yet) provide specific extensions for every OS-architecture combination that it supports. @umlaeute submitted a pull request for a consistent set of extensions (pure-data/pure-data@1247b31).

For the purpose of fat libraries, pdlibbuilder could define alternative extensions which:

1. are supported by Pd for quite a while, and
2. have a logical name that is not subject to debate

Of the existing alternatives, condition 2 excludes .l_ia64 and leaves the following :

.m_i386 for Windows / Intel 32 bit
.l_i386 for Linux / Intel 32 bit
.l_arm for Linux / ARM
.d_fat for OSX fat binaries
.d_ppc for OSX on PPC

Since OSX supports multiple architectures in a single binary and ppc can be considered obsolete (or included with fat binary), the first three in the list are most important in their function to distinguish architecture. Together with default extensions, up to 7 platforms could be supported in a fat library or project:

.m_i386 - Windows / Intel 32 bit
.dll - Windows / Intel 64 bit
.l_i386 - Linux / Intel 32 bit
.pd_linux - Linux / Intel 64 bit
.l_arm - Linux / ARM (32 or 64 bit)
.d_fat - OSX fat (Intel 32 and 64 bit)

This covers most of the platforms supported by Pd. It is currently not possible to distinguish between ARM 32 and 64 bit, or between ARM versions.