See http://postfix.1071664.n5.nabble.com/Support-for-quot-Linux-5-quot-td99786.html
--- a/makedefs	2018-02-03 12:20:46.000000000 -0500
+++ b/makedefs	2019-02-18 15:39:17.711376206 -0500
@@ -546,7 +546,7 @@
 		: ${SHLIB_ENV="LD_LIBRARY_PATH=`pwd`/lib"}
 		: ${PLUGIN_LD="${CC-gcc} -shared"}
 		;;
-  Linux.[34].*)	SYSTYPE=LINUX$RELEASE_MAJOR
+  Linux.[345].*)	SYSTYPE=LINUX$RELEASE_MAJOR
 		case "$CCARGS" in
 		 *-DNO_DB*) ;;
 		 *-DHAS_DB*) ;;
--- a/src/util/sys_defs.h	2019-02-18 15:40:19.008167828 -0500
+++ b/src/util/sys_defs.h	2019-02-18 15:42:17.241742169 -0500
@@ -748,7 +748,7 @@
  /*
   * LINUX.
   */
-#if defined(LINUX2) || defined(LINUX3) || defined(LINUX4)
+#if defined(LINUX2) || defined(LINUX3) || defined(LINUX4) || defined(LINUX5)
 #define SUPPORTED
 #define UINT32_TYPE	unsigned int
 #define UINT16_TYPE	unsigned short