autoconf: fix sonic/pcaudio definitions

Makefile.am

 src_libespeak_ng_la_CFLAGS = \
 	-fPIC -fvisibility=hidden \
 	-pedantic -fno-exceptions -DPATH_ESPEAK_DATA=\"$(DATADIR)\" -DLIBESPEAK_NG_EXPORT \
-	${PCAUDIOLIB_CFLAGS} ${AM_CFLAGS}
+	${PCAUDIOLIB_CFLAGS} ${SONIC_CFLAGS} ${AM_CFLAGS}
 
 src_libespeak_ng_la_SOURCES = \
 	src/ucd-tools/src/case.c \

configure.ac

     AC_CHECK_HEADERS([pcaudiolib/audio.h],
         [
             have_pcaudiolib=yes
-            PCAUDIOLIB_CFLAGS=
+            PCAUDIOLIB_CFLAGS=-DUSE_LIBPCAUDIO=1
             PCAUDIOLIB_LIBS=-lpcaudio
         ],[
             have_pcaudiolib=no
 else
     AC_CHECK_HEADERS([sonic.h],
         [
+            SONIC_CFLAGS=-DUSE_LIBSONIC=1
             AC_CHECK_LIB([sonic], [sonicCreateStream])
             have_sonic=yes
         ],[
             have_sonic=no
         ])
 fi
+AC_SUBST(SONIC_CFLAGS)
 
 AC_CHECK_HEADERS([valgrind/memcheck.h])