[cmake] Make setting of CMAKE_C(XX)_COMPILER flags overridable for cross-builds

Summary:
r319898 made it possible to override these variables via the
CROSS_TOOLCHAIN_FLAGS setting, but this only worked if one explicitly
specifies these variables there. If, instead, one uses
CROSS_TOOLCHAIN_FLAGS to specify a toolchain file (as our internal
builds do, to point cmake to a checked-in toolchain), the
CMAKE_C(XX)_COMPILER flags would still win over the ones specified by
the toolchain file.

To fix is to make the mere presence of these flags overridable. I do
this by putting them as a default value for the CROSS_TOOLCHAIN_FLAGS
setting, so they can be overridden at cmake configuration time.

Reviewers: hintonda, beanz

Subscribers: bogner, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D40947

llvm-svn: 320138

llvm/cmake/modules/CrossCompile.cmake

@@ -7,10 +7,16 @@ function(llvm_create_cross_target_internal target_name toolchain buildtype)
   endif(NOT DEFINED LLVM_${target_name}_BUILD)
 
   if (EXISTS ${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake)
-    set(CROSS_TOOLCHAIN_FLAGS_${target_name} 
-        -DCMAKE_TOOLCHAIN_FILE=\"${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake\"
-        CACHE STRING "Toolchain file for ${target_name}")
+    set(CROSS_TOOLCHAIN_FLAGS_INIT
+      -DCMAKE_TOOLCHAIN_FILE=\"${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake\")
+  else()
+    set(CROSS_TOOLCHAIN_FLAGS_INIT
+      -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
+      -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
+      )
   endif()
+  set(CROSS_TOOLCHAIN_FLAGS_${target_name} ${CROSS_TOOLCHAIN_FLAGS_INIT}
+    CACHE STRING "Toolchain configuration for ${target_name}")
 
   if (buildtype)
     set(build_type_flags "-DCMAKE_BUILD_TYPE=${buildtype}")
@@ -32,8 +38,6 @@ function(llvm_create_cross_target_internal target_name toolchain buildtype)
 
   add_custom_command(OUTPUT ${LLVM_${target_name}_BUILD}/CMakeCache.txt
     COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}"
-        -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
-        -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
         ${CROSS_TOOLCHAIN_FLAGS_${target_name}} ${CMAKE_SOURCE_DIR}
         -DLLVM_TARGET_IS_CROSSCOMPILE_HOST=TRUE
         -DLLVM_TARGETS_TO_BUILD=Native