feat(build): add 'CROSS_COMPILE' cmake option

When CROSS_COMPILE=ON, the Lean standard library will not be compiled.

.travis.windows.yml

@@ -24,7 +24,7 @@ script:
   - mkdir -p build
   - LEAN_ROOT=`pwd`
   - cd build
-  - cmake -DIGNORE_SORRY=ON -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE} ../src -DTCMALLOC=OFF -DCMAKE_TOOLCHAIN_FILE=/tmp/mxe/usr/x86_64-w64-mingw32/share/cmake/mxe-conf.cmake
+  - cmake -DCROSS_COMPILE=ON -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE} ../src -DTCMALLOC=OFF -DCMAKE_TOOLCHAIN_FILE=/tmp/mxe/usr/x86_64-w64-mingw32/share/cmake/mxe-conf.cmake
   - SITE=Windows@Travis
   - if [[ $CMAKE_BUILD_TYPE == RELEASE ]]; then
         BUILD_TYPE=Release;

src/CMakeLists.txt

@@ -24,6 +24,9 @@ option(JEMALLOC           "JEMALLOC"           OFF)
 # a version of Lean that does not report when 'sorry' is used.
 # This is useful for suppressing warning messages in the nightly builds.
 option(IGNORE_SORRY       "IGNORE_SORRY"      OFF)
+# When cross-compiling, we do not compile the standard library since
+# the executable will not work on the host machine
+option(CROSS_COMPILE      "CROSS_COMPILE"     OFF)
 
 # Added for CTest
 include(CTest)
@@ -322,13 +325,16 @@ if(NOT DEFINED PROCESSOR_COUNT)
   endif()
 endif()
 
-# Only build libraries if we are NOT cross compiling
-add_custom_target(
+if("${CROSS_COMPILE}" MATCHES "ON")
+  message(STATUS "Lean standard library will not be compiled when using cross-compilation.")
+else()
+  add_custom_target(
     standard_lib ALL
     COMMAND python ${LEAN_SOURCE_DIR}/../bin/linja
     DEPENDS ${CMAKE_BINARY_DIR}/shell/lean
     WORKING_DIRECTORY ${LEAN_SOURCE_DIR}/../library
     )
+endif()
 
 add_custom_target(clean-olean
   WORKING_DIRECTORY ${LEAN_SOURCE_DIR}/../library