Support for using alternate third party PortAudio, Speex, and GSM libraries

[bennylp]

Add support for using different PortAudio, Speex, and/or GSM library installation than the one in the third_party directory. This is useful to make use of specific features in the system that have been configured in the installed library. For example, PortAudio library that comes with OpenBSD has support for sndio, which is not normally available/enabled in our PortAudio.

New ./aconfigure options are to be implemented:

--with-external-speex   Use external Speex development files, not the one in
                        "third_party" directory. When this option is set,
                        make sure that Speex is accessible to use (hint: use
                        CFLAGS and LDFLAGS env var to set the include/lib
                        paths)
--with-external-gsm     Use external GSM codec library, not the one in
                        "third_party" directory. When this option is set,
                        make sure that the GSM include/lib files are
                        accessible to use (hint: use CFLAGS and LDFLAGS env
                        var to set the include/lib paths)
--with-external-pa      Use external PortAudio development files, not the
                        one in "third_party" directory. When this option is
                        set, make sure that PortAudio is accessible to use
                        (hint: use CFLAGS and LDFLAGS env var to set the
                        include/lib paths)

[bennylp]

In r3058:

• added --with-pa-path configure option
• remove the use of PaUtil?_SetDebugPrintFunction() since this is not always available across all PA versions (?)

[bennylp]

In r3062:

• added support for external libspeex and libgsm
• renamed --with-pa-path to --with-external-pa and changed the semantic a little bit; now we don't have to specify the path in the option, unless the library is not installed in the default location

[bennylp]

In r3064:

• fixed bug in r3062 above, which caused build error related to PortAudio in Mac OS X