COMP: suppress clang warnings about unknown compiler warning option

- allows enabling new warnings without causing messy output with older
  clang versions

wmake/rules/linux64Clang/c++

 SUFFIXES += .C .cc .cpp .cxx
 
-c++WARN     = -Wall -Wextra -Wold-style-cast -Wnon-virtual-dtor -Wno-unused-parameter -Wno-invalid-offsetof -Wno-undefined-var-template
+c++WARN     = \
+    -Wall -Wextra -Wold-style-cast \
+    -Wnon-virtual-dtor -Wno-unused-parameter -Wno-invalid-offsetof \
+    -Wno-undefined-var-template \
+    -Wno-unknown-warning-option
 
 # Suppress some warnings for flex++ and CGAL
-c++LESSWARN = -Wno-old-style-cast -Wno-unused-local-typedefs -Wno-tautological-undefined-compare -Wno-shift-negative-value
+c++LESSWARN = \
+    -Wno-old-style-cast -Wno-unused-local-typedefs \
+    -Wno-tautological-undefined-compare -Wno-shift-negative-value
 
 CC          = clang++ -std=c++11 -m64
 

wmake/rules/linuxARM64Clang/c++

 SUFFIXES += .C .cc .cpp .cxx
 
-c++WARN     = -Wall -Wextra -Wold-style-cast -Wnon-virtual-dtor -Wno-unused-parameter -Wno-invalid-offsetof -Wno-undefined-var-template
+c++WARN     = \
+    -Wall -Wextra -Wold-style-cast \
+    -Wnon-virtual-dtor -Wno-unused-parameter -Wno-invalid-offsetof \
+    -Wno-undefined-var-template \
+    -Wno-unknown-warning-option
 
 # Suppress some warnings for flex++ and CGAL
-c++LESSWARN = -Wno-old-style-cast -Wno-unused-local-typedefs -Wno-tautological-undefined-compare -Wno-shift-negative-value
+c++LESSWARN = \
+    -Wno-old-style-cast -Wno-unused-local-typedefs \
+    -Wno-tautological-undefined-compare -Wno-shift-negative-value
 
 ## CC          = clang++ -std=c++11 -m64
 CC          = clang++ -std=c++11

wmake/rules/linuxClang/c++

 SUFFIXES += .C .cc .cpp .cxx
 
-c++WARN     = -Wall -Wextra -Wold-style-cast -Wnon-virtual-dtor -Wno-unused-parameter -Wno-invalid-offsetof -Wno-undefined-var-template
+c++WARN     = \
+    -Wall -Wextra -Wold-style-cast \
+    -Wnon-virtual-dtor -Wno-unused-parameter -Wno-invalid-offsetof \
+    -Wno-undefined-var-template \
+    -Wno-unknown-warning-option
 
 # Suppress some warnings for flex++ and CGAL
-c++LESSWARN = -Wno-old-style-cast -Wno-unused-local-typedefs -Wno-tautological-undefined-compare -Wno-shift-negative-value
+c++LESSWARN = \
+    -Wno-old-style-cast -Wno-unused-local-typedefs \
+    -Wno-tautological-undefined-compare -Wno-shift-negative-value
 
 CC          = clang++ -std=c++11 -m32