more configure options explained

docs/INSTALL

@@ -130,6 +130,22 @@ UNIX
 
        ./configure --disable-shared
 
+     To tell the configure script to skip searching for thread-safe functions,
+     add an option like:
+
+       ./configure --disable-thread
+
+     To build curl with kerberos4 support enabled, curl requires the krb4 libs
+     and headers installed. You can then use a set of options to tell
+     configure where those are:
+
+          --with-krb4-includes[=DIR]   Specify location of kerberos4 headers
+          --with-krb4-libs[=DIR]       Specify location of kerberos4 libs
+          --with-krb4[=DIR]            where to look for Kerberos4
+
+     In most cases, /usr/athena is the install prefix and then it works with
+
+       ./configure --with-krb4=/usr/athena
 
 Win32
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