CMakeLists: Derive the source directory grouping from targets themselves

Removes the need to store to separate SRC and HEADER variables, and then
construct the target in most cases.

externals/getopt/CMakeLists.txt

@@ -1,11 +1,9 @@
-set(SRCS
-            getopt.c
-            )
-set(HEADERS
-            getopt.h
-            )
+add_library(getopt
+    getopt.c
+    getopt.h
+)
+
+create_target_directory_groups(getopt)
 
-create_directory_groups(${SRCS} ${HEADERS})
-add_library(getopt ${SRCS} ${HEADERS})
 target_compile_definitions(getopt PUBLIC STATIC_GETOPT)
 target_include_directories(getopt INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})