[R-SIG-Mac] Xcode 10.2.1

[Kasper Daniel Hansen]

I am on Mojave. I recently updated to Xcode 10.2.1 and I did the associated
update of the command line tools. Following this, I reinstalled clang6 and
gfortran 6 from CRAN.

When I use the clang6 compiler from CRAN, it is unable to find stdio.h.

When I compare the include path from clang6 to the Apple suppled clang with
`$CC -E - -v` I get

with Apple clang
#include <...> search starts here:
 /usr/local/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/10.0.1/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include
 /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/usr/include
 /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/System/Library/Frameworks
(framework directory)
End of search list.

with clang6 from CRAN
#include <...> search starts here:
 /usr/local/include
 /usr/local/clang6/lib/clang/6.0.0/include
 /System/Library/Frameworks (framework directory)
 /Library/Frameworks (framework directory)
End of search list.

Did Apple move the headers around? Or have I somehow hosed my system?

Best,
Kasper

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