build: cmake cleanup (#22251)

- Remove unused code
- Use consistent casing. Variable names such as LibLuV_LIBRARIES is
  needlessly jarring, even if the name might be technically correct.
- Use title casing for packages. find_package(unibilium) requires the
  find_module to be named "Findunibilium.cmake", which makes it harder
  to spot when scanning the files. Instead, use "Unibilium".

cmake/FindLibintl.cmake

@@ -0,0 +1,80 @@
+include(CheckCSourceCompiles)
+include(CheckVariableExists)
+
+# Append custom gettext path to CMAKE_PREFIX_PATH
+# if installed via Mac Homebrew
+if (APPLE)
+    find_program(HOMEBREW_PRG brew)
+    if (EXISTS ${HOMEBREW_PRG})
+        execute_process(COMMAND ${HOMEBREW_PRG} --prefix gettext
+            OUTPUT_STRIP_TRAILING_WHITESPACE
+            OUTPUT_VARIABLE HOMEBREW_GETTEXT_PREFIX)
+        list(APPEND CMAKE_PREFIX_PATH "${HOMEBREW_GETTEXT_PREFIX}")
+    endif()
+endif()
+
+find_path(LIBINTL_INCLUDE_DIR
+    NAMES libintl.h
+    PATH_SUFFIXES gettext
+)
+
+find_library(LIBINTL_LIBRARY
+    NAMES intl libintl
+)
+
+if (LIBINTL_INCLUDE_DIR)
+  list(APPEND CMAKE_REQUIRED_INCLUDES "${LIBINTL_INCLUDE_DIR}")
+endif()
+# On some systems (linux+glibc) libintl is passively available.
+# So only specify the library if one was found.
+if (LIBINTL_LIBRARY)
+  list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBINTL_LIBRARY}")
+endif()
+if (MSVC)
+  list(APPEND CMAKE_REQUIRED_LIBRARIES ${ICONV_LIBRARY})
+endif()
+
+# On macOS, if libintl is a static library then we also need
+# to link libiconv and CoreFoundation.
+get_filename_component(LibIntl_EXT "${LIBINTL_LIBRARY}" EXT)
+if (APPLE AND (LibIntl_EXT STREQUAL ".a"))
+  set(LibIntl_STATIC TRUE)
+  find_library(CoreFoundation_FRAMEWORK CoreFoundation)
+  list(APPEND CMAKE_REQUIRED_LIBRARIES "${ICONV_LIBRARY}" "${CoreFoundation_FRAMEWORK}")
+endif()
+
+check_c_source_compiles("
+#include <libintl.h>
+
+int main(int argc, char** argv) {
+  gettext(\"foo\");
+  ngettext(\"foo\", \"bar\", 1);
+  bindtextdomain(\"foo\", \"bar\");
+  bind_textdomain_codeset(\"foo\", \"bar\");
+  textdomain(\"foo\");
+}" HAVE_WORKING_LIBINTL)
+if (MSVC)
+  list(REMOVE_ITEM CMAKE_REQUIRED_LIBRARIES ${ICONV_LIBRARY})
+endif()
+if (LibIntl_STATIC)
+  list(REMOVE_ITEM CMAKE_REQUIRED_LIBRARIES  "${ICONV_LIBRARY}" "${CoreFoundation_FRAMEWORK}")
+endif()
+if (LIBINTL_INCLUDE_DIR)
+  list(REMOVE_ITEM CMAKE_REQUIRED_INCLUDES "${LIBINTL_INCLUDE_DIR}")
+endif()
+if (LIBINTL_LIBRARY)
+  list(REMOVE_ITEM CMAKE_REQUIRED_LIBRARIES "${LIBINTL_LIBRARY}")
+endif()
+
+set(REQUIRED_VARIABLES LIBINTL_LIBRARY LIBINTL_INCLUDE_DIR)
+if (HAVE_WORKING_LIBINTL)
+  # On some systems (linux+glibc) libintl is passively available.
+  # If HAVE_WORKING_LIBINTL then we consider the requirement satisfied.
+  unset(REQUIRED_VARIABLES)
+
+  check_variable_exists(_nl_msg_cat_cntr HAVE_NL_MSG_CAT_CNTR)
+endif()
+
+find_package_handle_standard_args(Libintl DEFAULT_MSG
+  ${REQUIRED_VARIABLES})
+mark_as_advanced(LIBINTL_LIBRARY LIBINTL_INCLUDE_DIR)