ExternalProject: Be compatible with projects setting CMAKE_CFG_INTDIR

Since commit v3.5.0-rc1~32^2~1 (ExternalProject: Simplify `cmake
--build` configuration passing, 2016-01-19) we use the `$<CONFIG>`
generator expression to generate the `cmake --build . --config <config>`
value for the default BUILD_COMMAND instead of the CMAKE_CFG_INTDIR
placeholder value provided by multi-config generators.  However, some
projects have been abusing the old implementation detail by setting
CMAKE_CFG_INTDIR themselves to get a specific configuration.  Those
projects should be updated to set their own BUILD_COMMAND to get
non-default behavior.  Meanwhile we can be compatible with their
existing releases by detecting when CMAKE_CFG_INTDIR is not a
generator-provided placeholder and using its value instead.

1 files changed, 17 insertions, 2 deletions

diff --git a/Modules/ExternalProject.cmake b/Modules/ExternalProject.cmake
index 7070dc4709..249658d620 100644
--- a/Modules/ExternalProject.cmake
+++ b/Modules/ExternalProject.cmake
@@ -1231,7 +1231,22 @@ function(_ep_get_build_command name step cmd_var)
         endif()
         set(args --build ".")
         if(CMAKE_CONFIGURATION_TYPES)
-          list(APPEND args --config $<CONFIG>)
+          if (CMAKE_CFG_INTDIR AND
+              NOT CMAKE_CFG_INTDIR STREQUAL "." AND
+              NOT CMAKE_CFG_INTDIR MATCHES "\\$")
+            # CMake 3.4 and below used the CMAKE_CFG_INTDIR placeholder value
+            # provided by multi-configuration generators.  Some projects were
+            # taking advantage of that undocumented implementation detail to
+            # specify a specific configuration here.  They should use
+            # BUILD_COMMAND to change the default command instead, but for
+            # compatibility honor the value.
+            set(config ${CMAKE_CFG_INTDIR})
+            message(AUTHOR_WARNING "CMAKE_CFG_INTDIR should not be set by project code.\n"
+              "To get a non-default build command, use the BUILD_COMMAND option.")
+          else()
+            set(config $<CONFIG>)
+          endif()
+          list(APPEND args --config ${config})
         endif()
         if(step STREQUAL "INSTALL")
           list(APPEND args --target install)
@@ -1241,7 +1256,7 @@ function(_ep_get_build_command name step cmd_var)
           string(REGEX REPLACE "^(.*/)cmake([^/]*)$" "\\1ctest\\2" cmd "${cmd}")
           set(args "")
           if(CMAKE_CONFIGURATION_TYPES)
-            list(APPEND args -C $<CONFIG>)
+            list(APPEND args -C ${config})
           endif()
         endif()
       endif()