cmake: treat building for another ISA as a cross-compile.

CMake appears to have the notion that a build is only a cross-compile if
the targt *operating system* is different.  This is an incorrect notion,
as even if the target is the *same* OS but a different instruction set,
you may not be able to do tests that involve compiling and running a
program.

Check whether CMAKE_GENERATOR_PLATFORM is set and has a value different
from that of CMAKE_HOST_SYSTEM_PROCESSOR and, if that's the case, set
CMAKE_CROSSCOMPILING to TRUE.

This comes from libpcap, where the equivalent change fixed issue
the-tcpdump-group/libpcap#1352.

(A different strategy may be necessary for cross-builds with UNIX
toolchains.)

(cherry picked from commit 2dd45475f02529c658aeb5a691dfb0c78e129aa1)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index b72b3df42bfdd656fa183386fcb71acd02d8ffcd..cd4a10726d33ea592bca0e6dc7fe57980f2df010 100644 (file)
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -399,6 +399,28 @@ if(MSVC)
    endif (USE_STATIC_RT)
 endif(MSVC)
 
+#
+# CMake's definition of "cross-compiling" appears to be "compiling
+# for an *operating system* other than the one on which the build
+# is being done*.
+#
+# This is an inadequate definition, as people build for the same
+# operating system but a different instruciton set, e.g. building
+# on an IA-32 or x86-64 Linux box for an Arm embedded Linux box,
+# or building Arm code on an IA-32 or x86-64 Windows box.
+#
+# At least for the Windows case, people may do those builds by
+# setting the target with th -A flag to CMake; that causes
+# CMAKE_GENERATOR_PLATFORM to be set to the target.  If
+# CMAKE_GENERATOR_PLATFORM is set, compare it with
+# CMAKE_HOST_SYSTEM_PROCESSOR and, if they're not equal, set
+# CMAKE_CROSSCOMPILING to TRUE.
+#
+if (CMAKE_GENERATOR_PLATFORM AND
+    NOT CMAKE_GENERATOR_PLATFORM STREQUAL CMAKE_HOST_SYSTEM_PROCESSOR)
+    set(CMAKE_CROSSCOMPILING TRUE)
+endif()
+
 ###################################################################
 #   Detect available platform features
 ###################################################################