Fix pthread missing in configure.ac

Hello,

The tarball of release v0.90.86 and the HEAD of git repository have the same problem : the compilation failed because some part of code requiring to be linked with pthread but it is not referenced on your autoconf rules.
The patch fix it.

../../src/ppui/osinterface/libosinterface.a(PPSystem_POSIX.o): dans la fonction « System::getTempFileName() »:
/home/cauldronuser/test/milkytracker/src/ppui/osinterface/posix/PPSystem_POSIX.cpp:64: AVERTISSEMENT: the use of `tmpnam' is dangerous, better use `mkstemp'
/usr/bin/ld: ../midi/libmidi.a(RtMidi.o): référence au symbole non défini «pthread_create@@GLIBC_2.1»
/lib/libpthread.so.0: error adding symbols: DSO missing from command line


Sincerly,