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.)

CMakeLists.txt

@@ -392,6 +392,28 @@ 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()
+
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 #   Detect available platform features
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