fix(opencv-test): guard on __linux__, drop build-phase define (gui-stack pattern)

The member BUILD phase (mcpp test --workspace runs build+test per member)
compiled tests/roundtrip.cpp with -DHAVE_OPENCV from [target.cfg(linux).build]
cxxflags but WITHOUT the cfg(linux) opencv dep's include path -> opencv2/core.hpp
not found. Match the gui-stack precedent: guard the test on the builtin __linux__
and drop the custom define + [target.build] section, so the build phase is a
clean no-op and the test phase compiles with the dep includes.

Verified from a clean state (fresh toolchain + fresh opencv source build,
released mcpp 0.0.87): mcpp build RC=0, mcpp test roundtrip ok.

Author: Sunrisepeak

tests/examples/opencv/mcpp.toml

@@ -13,6 +13,3 @@ compat = { path = "../../.." }
 
 [target.'cfg(linux)'.dependencies.compat]
 opencv = "4.13.0"
-
-[target.'cfg(linux)'.build]
-cxxflags = ["-DHAVE_OPENCV=1"]

tests/examples/opencv/tests/roundtrip.cpp

@@ -2,7 +2,7 @@
 // against the static libs built from source by the package's install() CMake hook.
 // Linux-only (see mcpp.toml); off-Linux this is a no-op main so `mcpp test
 // --workspace` is clean on macOS/Windows.
-#ifdef HAVE_OPENCV
+#ifdef __linux__
 #include <opencv2/core.hpp>
 #include <opencv2/imgproc.hpp>
 #include <opencv2/imgcodecs.hpp>