[flang] Allow configuring building the flang runtime standalone

[mstorsjo]

Summary

This allows building the flang runtime and libFortranDecimal standalone,
without setting up the full llvm build, by pointing cmake to these
directories.

This is the other one out of two essential steps for building the flang runtimes
for a foreign target, i.e. setting up cross compiling with flang.

[mstorsjo]

I should add here, that this is only a provisional first step. I guess the proper thing long term would be to split them out into separate toplevel projects (like libcxx), which then can be built with the llvm-projects/runtimes infrastructure, by listing the Fortran runtime in LLVM_ENABLE_RUNTIMES.

[rovka]

LGTM (based on skimming some other CMakeLists.txt around llvm). Maybe wait a bit before merging so others can have a look too.

flang/lib/Decimal/CMakeLists.txt

+if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+  cmake_minimum_required(VERSION 3.13.4)
+
+  project(FortranDecimal C CXX)
+
+  set(CMAKE_CXX_STANDARD 17)
+  set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
+  set(CMAKE_CXX_EXTENSIONS OFF)
+
+  set(FLANG_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../..")
+
+  set(LLVM_COMMON_CMAKE_UTILS "${FLANG_SOURCE_DIR}/../cmake")
+  set(LLVM_CMAKE_UTILS "${FLANG_SOURCE_DIR}/../llvm/cmake")
+
+  # Add path for custom modules
+  list(INSERT CMAKE_MODULE_PATH 0
+    "${FLANG_SOURCE_DIR}/cmake"
+    "${FLANG_SOURCE_DIR}/cmake/modules"
+    "${LLVM_COMMON_CMAKE_UTILS}"
+    "${LLVM_COMMON_CMAKE_UTILS}/Modules"
+    "${LLVM_CMAKE_UTILS}"
+    "${LLVM_CMAKE_UTILS}/modules"
+    )
+
+  include(AddLLVM)
+  include(AddFlang)
+
+  include(TestBigEndian)
+  test_big_endian(IS_BIGENDIAN)
+  if (IS_BIGENDIAN)
+    add_compile_definitions(FLANG_BIG_ENDIAN=1)
+  else ()
+    add_compile_definitions(FLANG_LITTLE_ENDIAN=1)
+  endif ()
+  include_directories(BEFORE
+    ${FLANG_SOURCE_DIR}/include)
+endif()
 
 add_flang_library(FortranDecimal INSTALL_WITH_TOOLCHAIN
   binary-to-decimal.cpp
   decimal-to-binary.cpp
 )

flang/runtime/CMakeLists.txt

 #===-- runtime/CMakeLists.txt ----------------------------------------------===#
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 #===------------------------------------------------------------------------===#
 
+if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+  cmake_minimum_required(VERSION 3.13.4)
+
+  project(FlangRuntime C CXX)
+
+  set(CMAKE_CXX_STANDARD 17)
+  set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
+  set(CMAKE_CXX_EXTENSIONS OFF)
+
+  set(FLANG_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/..")
+
+  set(LLVM_COMMON_CMAKE_UTILS "${FLANG_SOURCE_DIR}/../cmake")
+  set(LLVM_CMAKE_UTILS "${FLANG_SOURCE_DIR}/../llvm/cmake")
+
+  # Add path for custom modules
+  list(INSERT CMAKE_MODULE_PATH 0
+    "${FLANG_SOURCE_DIR}/cmake"
+    "${FLANG_SOURCE_DIR}/cmake/modules"
+    "${LLVM_COMMON_CMAKE_UTILS}"
+    "${LLVM_COMMON_CMAKE_UTILS}/Modules"
+    "${LLVM_CMAKE_UTILS}"
+    "${LLVM_CMAKE_UTILS}/modules"
+    )
+
+  include(AddLLVM)
+  include(AddFlang)
+
+  include(TestBigEndian)
+  test_big_endian(IS_BIGENDIAN)
+  if (IS_BIGENDIAN)
+    add_compile_definitions(FLANG_BIG_ENDIAN=1)
+  else ()
+    add_compile_definitions(FLANG_LITTLE_ENDIAN=1)
+  endif ()
+  include_directories(BEFORE
+    ${FLANG_SOURCE_DIR}/include)
+endif()
+
 include(CheckCXXSymbolExists)
 include(CheckCXXSourceCompiles)
 check_cxx_symbol_exists(strerror string.h HAVE_STRERROR)
 check_cxx_symbol_exists(strerror_r string.h HAVE_STRERROR_R)
 # Can't use symbol exists here as the function is overloaded in C++
 check_cxx_source_compiles(
   "#include <string.h>
    int main() {