Strict option checking unless otherwise specified

Having option checking catches typos. Disabling the checking enable us
to use the same build options for a bunch of packages even though they
may not have the same dependencies.

configure

@@ -19,6 +19,7 @@ Optional Features:
   --disable-silent-rules  print every compilation statement as executed
   --enable-system-debug   put .debug files in global GDB debug dir
                           [default=yes if prefix=/usr, no otherwise]
+  --disable-option-checking  ignore unrecognized --enable/--with options
 
 Optional Packages:
   --with-dune-common=PATH use DUNE-common library from a specified location
@@ -65,6 +66,14 @@ Try \`$0 --help' for more information
 EOF
 }
 
+# exit only if option checking is enabled, otherwise ignore
+invalid_opt () {
+  if [ "${option_check}" = "yes" ]; then
+    invalid_arg $@
+    exit 1
+  fi
+}
+
 # default values
 prefix=/usr/local
 buildtype=Debug
@@ -74,6 +83,7 @@ pch_use=
 silent_rules=
 #debug_loc=" -DSYSTEM_DEBUG=OFF"
 debug_loc=
+option_check=yes
 
 # this variable will get feature options
 FEATURES=
@@ -130,10 +140,12 @@ while getopts -- ":-:" optchar; do
               rootvar="${pkgname^^}_PREFIX"
               ;;
             *)
-              rootvar="${pkgname}_DIR"
+              invalid_opt --with-${pkgname}
+              rootvar=""
               ;;
           esac
           # add this to the list of existing features
+          test -n "${rootvar}" && \
           FEATURES="${FEATURES} \"-D${rootvar}=${pkgloc}\""
           ;;
         disable-*)
@@ -147,6 +159,11 @@ while getopts -- ":-:" optchar; do
           esac
           # casing is of course different
           case "${pkgname}" in
+            option-checking)
+              option_check=no
+              # special flag: don't disable any particular package
+              pkgname=""
+              ;;
             debug)
               buildtype=Release
               # special flag: don't disable any particular package
@@ -175,6 +192,10 @@ while getopts -- ":-:" optchar; do
             gxx11check)
               pkgname="CXX11Features"
               ;;
+            *)
+              invalid_opt --disable-${pkgname}
+              pkgname=""
+              ;;
           esac
           # only disable packages if the flag refers to a proper one
           test -n "${pkgname}" && \
@@ -201,8 +222,8 @@ while getopts -- ":-:" optchar; do
               shared="OFF"
               ;;
             *)
-              invalid_arg --enable-${kind}
-              exit 1
+              invalid_opt --enable-${kind}
+              shared=""
               ;;
           esac
           test -n "${shared}" && \