build: unset C{,PLUS}_INCLUDE_PATH to avoid cross-toolchain conflict

macOS cross builds need `C{,PLUS}_INCLUDE_PATH` for Qt's native tooling
but causes issues when there are conflicting definitions between the
native and cross toolchains

Author: kwvg

depends/hosts/darwin.mk

@@ -51,11 +51,11 @@ darwin_STRIP=$(shell $(SHELL) $(.SHELLFLAGS) "command -v llvm-strip")
 #         Disable adhoc codesigning (for now) when using LLVM tooling, to avoid
 #         non-determinism issues with the Identifier field.
 
-darwin_CC=$(clang_prog) --target=$(host) \
+darwin_CC=env -u C_INCLUDE_PATH -u CPLUS_INCLUDE_PATH $(clang_prog) --target=$(host) \
               -isysroot$(OSX_SDK) -nostdlibinc \
               -iwithsysroot/usr/include -iframeworkwithsysroot/System/Library/Frameworks
 
-darwin_CXX=$(clangxx_prog) --target=$(host) \
+darwin_CXX=env -u C_INCLUDE_PATH -u CPLUS_INCLUDE_PATH $(clangxx_prog) --target=$(host) \
                -isysroot$(OSX_SDK) -nostdlibinc \
                -iwithsysroot/usr/include/c++/v1 \
                -iwithsysroot/usr/include -iframeworkwithsysroot/System/Library/Frameworks