Don't require busted.

Only provide the unittest target if busted was found.  And only build
nvim-test if the unittest target exists by excluding nvim-test from all.
Note: this means nvim-test won't be built by default, but it will be
built when you try to run unittests.

CMakeLists.txt

@@ -88,6 +88,7 @@ if(NOT BUSTED_OUTPUT_TYPE)
   set(BUSTED_OUTPUT_TYPE "utf_terminal")
 endif()
 
+if(BUSTED_PRG)
   get_target_property(NVIM_TEST_LIB nvim-test LOCATION)
   add_custom_target(unittest
     COMMAND ${CMAKE_COMMAND}
@@ -100,6 +101,7 @@ add_custom_target(unittest
       -DTEST_INCLUDES=${CMAKE_BINARY_DIR}/test/includes/post
       -P ${CMAKE_MODULE_PATH}/RunUnittests.cmake
     DEPENDS nvim-test unittest-headers)
+endif()
 
 # Unfortunately, the below does not work under Ninja.  Ninja doesn't use a
 # pseudo-tty when launching processes, because it can put many jobs in parallel

src/CMakeLists.txt

@@ -55,7 +55,7 @@ if(NOT DEFINED ENV{SKIP_EXEC})
 endif()
 
 if(NOT DEFINED ENV{SKIP_UNITTEST})
-  add_library(nvim-test MODULE ${NEOVIM_SOURCES} ${OS_SOURCES})
+  add_library(nvim-test MODULE EXCLUDE_FROM_ALL ${NEOVIM_SOURCES} ${OS_SOURCES})
   target_link_libraries(nvim-test ${NVIM_LINK_LIBRARIES})
 endif()