fix --disable-doxygen and --disable-c++17

Reported by Yuri Victorovich.

* configure.ac (enable_doxygen, enable_c++17): Use $enableval instead
of "yes".
* THANKS: Add Yuri.

THANKS

@@ -42,3 +42,4 @@ Tomáš Babiak
 Valentin Iovene
 Vitus Lam
 Yann Thierry-Mieg
+Yuri Victorovich

configure.ac

@@ -51,12 +51,12 @@ AM_SILENT_RULES([no])
 AC_ARG_ENABLE([c++17],
               [AC_HELP_STRING([--enable-c++17],
                               [Compile in C++17 mode.])],
-              [enable_17=yes], [enable_17=no])
+              [enable_17=$enableval], [enable_17=no])
 
 AC_ARG_ENABLE([doxygen],
               [AC_HELP_STRING([--enable-doxygen]),
   [enable generation of Doxygen documentation (requires Doxygen)])],
-  [enable_doxygen=yes], [enable_doxygen=no])
+  [enable_doxygen=$enableval], [enable_doxygen=no])
 AM_CONDITIONAL([ENABLE_DOXYGEN], [test "x${enable_doxygen:-no}" = xyes])
 
 # Option to indicate the maximal number of acceptance marks