Ensure -lpthread will be used when linking statically on linux

configure.ac

@@ -46,6 +46,8 @@ case "${host_os}" in
   linux*)
     SYS=linux
     CXXFLAGS="${CXXFLAGS} -pthread"
+dnl Specify -lpthread as part of Libs.private, in case of a static build
+    THREAD_LIBS="-lpthread"
     ;;
   bsdi*)
     SYS=bsdi
@@ -326,4 +328,6 @@ Makefile
 medialibrary.pc
 ])
 
+AC_SUBST(THREAD_LIBS)
+
 AC_OUTPUT

medialibrary.pc.in

@@ -8,4 +8,4 @@ Description: cross platform medialibrary
 Version: @PACKAGE_VERSION@
 Cflags: -I${includedir}
 Libs: -L${libdir} -lmedialibrary
-Libs.private: -lsqlite3 @LIBJPEG_LIBS@
+Libs.private: -lsqlite3 @LIBJPEG_LIBS@ @THREAD_LIBS@