build/CMake: find_package(… REQUIRED)

"Always use `find_package` with `REQUIRED`."

- We make an exception for LuaJit (not REQUIRED): the `nvim-test` target
  is included only if we can find LuaJit.

This is partially a cargo-cult (reference below), but it uncovered at
least one problem: `find_package(LibIntl REQUIRED)` fails on my vanilla
ubuntu 16.04 system.

ref: https://schneide.blog/2017/11/06/4-tips-for-better-cmake/

> optional dependencies is nice, but skipping on REQUIRED is not the way
> you want to do it. In the worst case, some of your features will just
> not work if those packages are not found, with no explanation
> whatsoever. Instead, use explicit feature-toggles (e.g. using option())
> that either skip the find_package call or use it with REQUIRED, so the
> user will know that another lib is needed for this feature.

src/nvim/CMakeLists.txt

@@ -484,7 +484,9 @@ set_property(
   APPEND_STRING PROPERTY COMPILE_FLAGS " -DMAKE_LIB "
 )
 
-if(LUAJIT_FOUND)
+if(NOT LUAJIT_FOUND)
+  message(STATUS "luajit not found, skipping nvim-test (unit tests) target")
+else()
   set(NVIM_TEST_LINK_LIBRARIES ${NVIM_LINK_LIBRARIES} ${LUAJIT_LIBRARIES})
   add_library(
     nvim-test

src/nvim/po/CMakeLists.txt

@@ -1,4 +1,4 @@
-find_package(Gettext)
+find_package(Gettext REQUIRED)
 find_program(XGETTEXT_PRG xgettext)
 find_program(ICONV_PRG iconv)