[libc] warns about missing linting only in full build mode

[gchatelet]

Summary

This is a follow up on https://reviews.llvm.org/D101528

[sivachandra]

LGTM with one additional suggestion.

libc/CMakeLists.txt

47

May be add something like this below this line:

if(LLVM_LIBC_ENABLE_LINTING AND (NOT LLVM_LIBC_FULL_BUILD))
  message(FATAL_ERROR "Cannot enable linting when full libc build is not enabled.")
endif()

Linting, as implemented currently, is meaningless when not using LLVM libc headers.

[gchatelet]

• Add error when linting is ON but full build is disabled.

[gchatelet]

This causes at least one builder to fail so it's probably is misconfiguration
https://lab.llvm.org/buildbot/#/builders/138/builds/4406

libc/CMakeLists.txt

if(COMMAND_RETURN_CODE EQUAL 0)
   message(STATUS "Set COMPILER_RESOURCE_DIR to "
                  "${COMPILER_RESOURCE_DIR} using --print-resource-dir")
 else()
   set(COMPILER_RESOURCE_DIR OFF)
   message(STATUS "COMPILER_RESOURCE_DIR not set
                   --print-resource-dir not supported by host compiler")
 endif()
 
+option(LLVM_LIBC_FULL_BUILD "Build and test LLVM libc as if it is the full libc" OFF)
+
 option(LLVM_LIBC_ENABLE_LINTING "Enables linting of libc source files" OFF)
 if(LLVM_LIBC_ENABLE_LINTING)
   if("clang-tools-extra" IN_LIST LLVM_ENABLE_PROJECTS
              AND "clang" IN_LIST LLVM_ENABLE_PROJECTS)
     add_custom_target(lint-libc)
   else()
     message(FATAL_ERROR "
       'clang' and 'clang-tools-extra' are required in LLVM_ENABLE_PROJECTS to
       lint llvm-libc. The linting step performs important checks to help prevent
       the introduction of subtle bugs, but it may increase build times.
 
       To disable linting set LLVM_LIBC_ENABLE_LINTING to OFF
       (pass -DLLVM_LIBC_ENABLE_LINTING=OFF to cmake).")
   endif()
-else()
+elseif(LLVM_LIBC_FULL_BUILD)
   message(WARNING "
     Linting for libc is currently disabled.
 
     This is not recommended, to enable set LLVM_LIBC_ENABLE_LINTING to ON
     (pass -DLLVM_LIBC_ENABLE_LINTING=ON to cmake).")
 endif()
 
-option(LLVM_LIBC_FULL_BUILD "Build and test LLVM libc as if it is the full libc" OFF)
-
 include(CMakeParseArguments)
 include(LLVMLibCRules)
 include(LLVMLibCCheckCpuFeatures)
 
 include("${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/${LIBC_TARGET_MACHINE}/entrypoints.txt")
 include("${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/${LIBC_TARGET_MACHINE}/headers.txt")
 
 set(TARGET_ENTRYPOINT_NAME_LIST "")