Do not treat <strings.h> as an umbrella header for <string.h>
The two are distinct on GNU/Linux.
    
Also add missing #include <string.h> in some files.

diff --git a/src/sed/lib/getline.c b/src/sed/lib/getline.c
index 4cb9d45b..f39c363f 100644
--- a/src/sed/lib/getline.c
+++ b/src/sed/lib/getline.c
@@ -9,10 +9,10 @@
 #include <stdio.h>
 #undef getline 			/* bird */
 
+#include <string.h>
+
 #ifdef HAVE_STRINGS_H
 # include <strings.h>
-#else
-# include <string.h>
 #endif /* HAVE_STRINGS_H */
 
 #ifdef HAVE_STDLIB_H
diff --git a/src/sed/lib/mkstemp.c b/src/sed/lib/mkstemp.c
index 5b00205c..f36f7ca4 100644
--- a/src/sed/lib/mkstemp.c
+++ b/src/sed/lib/mkstemp.c
@@ -2,10 +2,10 @@
 #include "config.h"
 #endif
 
+#include <string.h>
+
 #ifdef HAVE_STRINGS_H
 # include <strings.h>
-#else
-# include <string.h>
 #endif /* HAVE_STRINGS_H */
 
 #ifdef HAVE_STDLIB_H
diff --git a/src/sed/lib/utils.c b/src/sed/lib/utils.c
index 41c61aaf..48a8b57f 100644
--- a/src/sed/lib/utils.c
+++ b/src/sed/lib/utils.c
@@ -25,10 +25,10 @@
   extern int errno;
 #endif
 
+#include <string.h>
+
 #ifdef HAVE_STRINGS_H
 # include <strings.h>
-#else
-# include <string.h>
 #endif /* HAVE_STRINGS_H */
 
 #ifdef HAVE_STDLIB_H
diff --git a/src/sed/sed/compile.c b/src/sed/sed/compile.c
index 0ecc2fe2..064bb350 100644
--- a/src/sed/sed/compile.c
+++ b/src/sed/sed/compile.c
@@ -22,14 +22,13 @@
 #include "strverscmp.h"
 #include <stdio.h>
 #include <ctype.h>
+#include <string.h>
 
 #ifdef HAVE_STRINGS_H
 # include <strings.h>
 # ifdef HAVE_MEMORY_H
 #  include <memory.h>
 # endif
-#else
-# include <string.h>
 #endif /* HAVE_STRINGS_H */
 
 #ifdef HAVE_STDLIB_H
diff --git a/src/sed/sed/execute.c b/src/sed/sed/execute.c
index c9038f4f..09080b5b 100644
--- a/src/sed/sed/execute.c
+++ b/src/sed/sed/execute.c
@@ -24,6 +24,7 @@
 
 #include <stdio.h>
 #include <ctype.h>
+#include <string.h>
 
 #include <errno.h>
 #ifndef errno
@@ -46,8 +47,6 @@ extern int errno;
 
 #ifdef HAVE_STRINGS_H
 # include <strings.h>
-#else
-# include <string.h>
 #endif /*HAVE_STRINGS_H*/
 #ifdef HAVE_MEMORY_H
 # include <memory.h>
diff --git a/src/sed/sed/fmt.c b/src/sed/sed/fmt.c
index 64600a06..4374070e 100644
--- a/src/sed/sed/fmt.c
+++ b/src/sed/sed/fmt.c
@@ -22,6 +22,7 @@
 #include <stdio.h>
 #include <ctype.h>
 #include <sys/types.h>
+#include <string.h>
 
 #if HAVE_LIMITS_H
 # include <limits.h>
diff --git a/src/sed/sed/mbcs.c b/src/sed/sed/mbcs.c
index 37565472..79e3f843 100644
--- a/src/sed/sed/mbcs.c
+++ b/src/sed/sed/mbcs.c
@@ -17,6 +17,7 @@
 
 #include "sed.h"
 #include <stdlib.h>
+#include <string.h>
 
 int mb_cur_max;
 
diff --git a/src/sed/sed/regexp.c b/src/sed/sed/regexp.c
index d4e7437c..a007494d 100644
--- a/src/sed/sed/regexp.c
+++ b/src/sed/sed/regexp.c
@@ -19,6 +19,7 @@
 
 #include <ctype.h>
 #include <stdio.h>
+#include <string.h>
 #ifdef HAVE_STDLIB_H
 # include <stdlib.h>
 #endif
diff --git a/src/sed/sed/sed.c b/src/sed/sed/sed.c
index b3bc2ea2..3829e2c6 100644
--- a/src/sed/sed/sed.c
+++ b/src/sed/sed/sed.c
@@ -22,12 +22,10 @@
 
 #include "sed.h"
 
-
 #include <stdio.h>
+#include <string.h>
 #ifdef HAVE_STRINGS_H
 # include <strings.h>
-#else
-# include <string.h>
 #endif /*HAVE_STRINGS_H*/
 #ifdef HAVE_MEMORY_H
 # include <memory.h>