TARGET_OS_OSX not defined

Building the latest release on for the conda feedstock, here https://github.com/conda-forge/catch2-feedstock/pull/37 , gives the following error on OSX:

```
[3/120] Building CXX object projects/CMakeFiles/SelfTest.dir/SelfTest/IntrospectiveTests/PartTracker.tests.cpp.o
FAILED: projects/CMakeFiles/SelfTest.dir/SelfTest/IntrospectiveTests/PartTracker.tests.cpp.o
$BUILD_PREFIX/bin/x86_64-apple-darwin13.4.0-clang++ -I../include -march=core2 -mtune=haswell -mssse3 -ftree-vectorize -fPIC -fPIE -fstack-protector-strong -O2 -pipe -stdlib=libc++ -fvisibility-inlines-hidden -std=c++14 -fmessage-length=0 -isystem $PREFIX/include -fdebug-prefix-map=$SRC_DIR=/usr/local/src/conda/catch2-2.13.4 -fdebug-prefix-map=$PREFIX=/usr/local/src/conda-prefix -O3 -DNDEBUG -isysroot /Applications/Xcode_12.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk -mmacosx-version-min=10.9 -Wall -Wextra -Wunreachable-code -Wpedantic -Wmissing-declarations -Wweak-vtables -Wexit-time-destructors -Wglobal-constructors -Wmissing-noreturn -MD -MT projects/CMakeFiles/SelfTest.dir/SelfTest/IntrospectiveTests/PartTracker.tests.cpp.o -MF projects/CMakeFiles/SelfTest.dir/SelfTest/IntrospectiveTests/PartTracker.tests.cpp.o.d -o projects/CMakeFiles/SelfTest.dir/SelfTest/IntrospectiveTests/PartTracker.tests.cpp.o -c ../projects/SelfTest/IntrospectiveTests/PartTracker.tests.cpp
In file included from ../projects/SelfTest/IntrospectiveTests/PartTracker.tests.cpp:9:
In file included from ../include/internal/catch_test_case_tracker.h:11:
In file included from ../include/internal/catch_compiler_capabilities.h:28:
../include/internal/catch_platform.h:14:6: error: 'TARGET_OS_OSX' is not defined, evaluates to 0 [-Werror,-Wundef-prefix=TARGET_OS_OSX]
# if TARGET_OS_OSX == 1
^
1 error generated.
```

Start with include/internal/catch_platform.h and inspect the TARGET_OS_OSX conditional reported by the SelfTest compilation. Reproduce the conda-forge macOS build or the affected SelfTest target, then verify that compilation completes without the undefined-macro diagnostic.

Written by the indexing model from the issue text.