- mentioned needed -for installation- header files in README.

README.org

@@ -88,6 +88,15 @@ manager of your distribution (~apt~, ~yum~ etc).
    - libssl  (TLS socket)
    - linidn2 (i18n domain name aka idna)
 
+     The  C library  should  be installed  with  their header  (~*.h~)
+     files.  On Debian  or derived  system this  means installing  the
+     library package with the ~-dev~ suffix as shown below:
+
+     #+BEGIN_SRC sh
+     # apt-get install libssl-dev libidn2-dev
+     #+END_SRC
+
+
 * Install
 
   1. optional step needed only if  you have not already the configure script,
@@ -132,7 +141,7 @@ manager of your distribution (~apt~, ~yum~ etc).
   6. install on your system:
 
      #+BEGIN_SRC sh
-     $ make install
+     # make install
      #+END_SRC
 
   7. take a look the manpage:

README.txt

@@ -130,6 +130,14 @@ Table of Contents
   • libssl (TLS socket)
   • linidn2 (i18n domain name aka idna)
 
+    The C library should be installed with their header (`*.h') files.
+    On Debian or derived system this means installing the library
+    package with the `-dev' suffix as shown below:
+
+    ┌────
+    │ # apt-get install libssl-dev libidn2-dev
+    └────
+
 
 4 Install
 ═════════
@@ -177,7 +185,7 @@ Table of Contents
   6. install on your system:
 
      ┌────
-     │ $ make install
+     │ # make install
      └────
 
   7. take a look the manpage: