Build system fixes.

libtoxcore.pc now has -lpthread like it's supposed to on systems
where it is needed.

configure.ac

@@ -489,6 +489,13 @@ AX_PTHREAD(
     ]
 )
 
+AC_CHECK_LIB([pthread], [pthread_self],
+    [
+        PTHREAD_LDFLAGS="-lpthread"
+        AC_SUBST(PTHREAD_LDFLAGS)
+    ]
+)
+
 if test "x$BUILD_AV" = "xyes"; then
     PKG_CHECK_MODULES([OPUS], [opus],
         [],
@@ -512,8 +519,7 @@ fi
 if test "x$BUILD_AV" = "xyes"; then
     # toxcore lib needs an global?
     # So far this works okay
-    ## What about pthread?
-    AV_LIBS="$OPUS_LIBS $VPX_LIBS -pthread"
+    AV_LIBS="$OPUS_LIBS $VPX_LIBS"
     AC_SUBST(AV_LIBS)
 
     AV_CFLAGS="$OPUS_CFLAGS $VPX_CFLAGS"

libtoxav.pc.in

@@ -5,7 +5,7 @@ includedir=@includedir@
 
 Name: libtoxav
 Description: Tox A/V library
-Requires:
+Requires: libtoxcore
 Version: @PACKAGE_VERSION@
 Libs: -L${libdir} -ltoxav @AV_LIBS@
-Cflags: -I${includedir}
+Cflags: -I${includedir}

libtoxcore.pc.in

@@ -7,5 +7,5 @@ Name: libtoxcore
 Description: Tox protocol library
 Requires:
 Version: @PACKAGE_VERSION@
-Libs: @NACL_OBJECTS_PKGCONFIG@ -L${libdir} @NACL_LDFLAGS@ -ltoxdns -ltoxencryptsave -ltoxcore @NACL_LIBS@ @LIBS@ @MATH_LDFLAGS@
+Libs: @NACL_OBJECTS_PKGCONFIG@ -L${libdir} @NACL_LDFLAGS@ -ltoxdns -ltoxencryptsave -ltoxcore @NACL_LIBS@ @LIBS@ @MATH_LDFLAGS@ @PTHREAD_LDFLAGS@
 Cflags: -I${includedir}