Stop manually setting CMAKE_CROSSCOMPILING

This is set implicitly when CMAKE_SYSTEM_NAME is set. According to
the docs, the user isn't supposed to set it manually.

build-libcxx.sh

@@ -85,7 +85,6 @@ for arch in $ARCHS; do
         -DCMAKE_C_COMPILER=$arch-w64-mingw32-clang \
         -DCMAKE_CXX_COMPILER=$arch-w64-mingw32-clang++ \
         -DCMAKE_CXX_COMPILER_TARGET=$arch-w64-windows-gnu \
-        -DCMAKE_CROSSCOMPILING=TRUE \
         -DCMAKE_SYSTEM_NAME=Windows \
         -DCMAKE_C_COMPILER_WORKS=TRUE \
         -DCMAKE_CXX_COMPILER_WORKS=TRUE \

build-lldb-mi.sh

@@ -92,7 +92,6 @@ if [ -n "$HOST" ]; then
     BUILDDIR=$BUILDDIR-$HOST
 
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_SYSTEM_NAME=Windows"
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CROSSCOMPILING=TRUE"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_C_COMPILER=$HOST-gcc"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CXX_COMPILER=$HOST-g++"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_RC_COMPILER=$HOST-windres"

build-llvm.sh

@@ -148,7 +148,6 @@ CMAKEFLAGS="$LLVM_CMAKEFLAGS"
 
 if [ -n "$HOST" ]; then
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_SYSTEM_NAME=Windows"
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CROSSCOMPILING=TRUE"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_C_COMPILER=$HOST-gcc"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CXX_COMPILER=$HOST-g++"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_RC_COMPILER=$HOST-windres"

build-openmp.sh

@@ -80,7 +80,6 @@ for arch in $ARCHS; do
         -DCMAKE_CXX_COMPILER=$arch-w64-mingw32-clang++ \
         -DCMAKE_RC_COMPILER=$arch-w64-mingw32-windres \
         -DCMAKE_ASM_MASM_COMPILER=llvm-ml \
-        -DCMAKE_CROSSCOMPILING=TRUE \
         -DCMAKE_SYSTEM_NAME=Windows \
         -DCMAKE_AR="$PREFIX/bin/llvm-ar" \
         -DCMAKE_RANLIB="$PREFIX/bin/llvm-ranlib" \