warning: overriding '-mmacosx-version-min=10.14' option

[Jacob Carlborg]

When compiling the runtime for iOS using `ldc-build-runtime` using the 
following command:

ldc-build-runtime \
--cFlags='-target arm64-apple-ios12.0;-isysroot $IOS_SDK_PATH' \
--linkerFlags='-target arm64-apple-ios12.0;-isysroot $IOS_SDK_PATH' \
--dFlags='-mtriple=arm64-apple-ios12.0;-w;-de;-dip1000' \
--ldc=~/ldc/build/bin/ldc2 \
--ldcSrcDir=~/ldc \
--ninja \
--testrunners

I get warnings like this:

clang: warning: overriding '-mmacosx-version-min=10.14' option with 
'--target=arm64-apple-ios12.0' [-Woverriding-t-option]

Looking at the `build.ninja` file that is generated there are 72 targets 
(or whatever they're called) that contains the following:

FLAGS = -DHAVE_UNISTD_H -isysroot $MAC_OSX_SDK_PATH 
-mmacosx-version-min=10.14   -target arm64-apple-ios12.0 -isysroot 
$IOS_SDK_PATH

(I've replaced the actual SDK paths with $MAC_OSX_SDK_PATH and 
$IOS_SDK_PATH to reduce the text).

-- 
/Jacob Carlborg