diff -Naruw pgpool-II-4.2.0.orig/configure.ac pgpool-II-4.2.0/configure.ac
--- pgpool-II-4.2.0.orig/configure.ac	2020-11-25 20:25:42.000000000 -0500
+++ pgpool-II-4.2.0/configure.ac	2020-12-16 06:40:11.150611861 -0500
@@ -4,6 +4,10 @@
 dnl Checks for programs.
 AC_PROG_CC
 AC_PROG_CC_C99()
+AX_PTHREAD
+LIBS="$PTHREAD_LIBS $LIBS"
+CFLAGS="$CFLAGS $PTHREAD_CFLAGS"
+CC="$PTHREAD_CC"
 
 # Error out if the compiler does not support C99, as the codebase
 # relies on that.