FS-3638: Rework platform library/linker flags handling and libfreeswitch _LDFLAGS/_LIBS to fix applications

This fixes the linking order problems reported in FS-3638.

Clean up libfreeswitch dependency handling:

        PLATFORM_CORE_LDFLAGS - Non-library linker flags, specific to this platform
        PLATFORM_CORE_LIBS    - Library flags/libraries need to build libfreeswitch on this platform

Drop LDADD dependencies from tone2wav and fs_encode, except for libfreeswitch.la, which will
"automagically" pull in all other libraries, now that we fixed libfreeswitch.
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>