fix: set CXXFLAGS/SDKROOT so boring-sys compiles on macOS 26 (Tahoe) (#138)

macOS 26 moved libc++ headers inside the SDK, breaking Apple clang 17's
default header search. The boring-sys cmake step fails with
'fatal error: memory file not found' when building --features stealth.

Adding CXXFLAGS and SDKROOT to .cargo/config.toml [env] propagates them
to all build scripts (Cargo's documented mechanism), so cmake picks up the
SDK's C++ include path without any manual env-var ceremony.

force=false preserves CI overrides and Xcode users who already have
SDKROOT set. Non-macOS platforms ignore SDKROOT; clang silently skips
-isystem paths that do not exist on disk.

Fixes #136

.cargo/config.toml

@@ -1,2 +1,12 @@
 [env]
 RUST_TEST_THREADS = "1"
+
+# macOS 26 (Tahoe) moved libc++ headers inside the SDK; Apple clang 17 no longer
+# finds them at the old search path. This causes boring-sys's cmake step to fail
+# when building with --features stealth. Setting CXXFLAGS and SDKROOT here makes
+# them visible to all build scripts (including boring-sys's) so cmake can locate
+# the headers. force=false lets CI or developers override via their shell env.
+# Non-macOS platforms ignore SDKROOT; the -isystem path is silently skipped if
+# it doesn't exist. See: https://github.com/h4ckf0r0day/obscura/issues/136
+CXXFLAGS = { value = "-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1", force = false }
+SDKROOT  = { value = "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk", force = false }