luajit fails to build on macOS 10.14 Mojave

[fxcoudert]

Part of #28817 . luajit fails to build on macOS 10.14 Mojave. During the make amalg step, it errors out with:

HOSTLINK  host/minilua
ld: library not found for -lgcc_s.10.4
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [host/minilua] Error 1

macOS 10.14 / Xcode 10 have removed libgcc_s.10.4, but I do not know why this is called because nothing specifies -mmacosx-min. The clang command is the following:

clang called with: -o host/minilua host/minilua.o -lm
superenv added:    -pipe -w -O2 -march=native -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk --sysroot=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk -isystem/usr/local/include -isystem/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/usr/include/libxml2 -isystem/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/OpenGL.framework/Versions/Current/Headers -L/usr/local/lib -L/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/OpenGL.framework/Versions/Current/Libraries -Wl,-headerpad_max_install_names
superenv executed: clang -pipe -w -O2 -march=native -o host/minilua host/minilua.o -lm -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk --sysroot=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk -isystem/usr/local/include -isystem/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/usr/include/libxml2 -isystem/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/OpenGL.framework/Versions/Current/Headers -L/usr/local/lib -L/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/OpenGL.framework/Versions/Current/Libraries -Wl,-headerpad_max_install_names

What is very weird is that actually running this exact command shown above, from the shell, succeeds in building host/minilua correctly.

[fxcoudert]

The bug can be reproduced outside of Homebrew and superenv:

einstein:LuaJIT-2.0.5 fx$ make amalg
Building LuaJIT 2.0.5
/Applications/Xcode.app/Contents/Developer/usr/bin/make -C src amalg
+--------------------------------------------------------------------------+
| WARNING: Compiling the amalgamation needs a lot of virtual memory        |
| (around 300 MB with GCC 4.x)! If you don't have enough physical memory   |
| your machine will start swapping to disk and the compile will not finish |
| within a reasonable amount of time.                                      |
| So either compile on a bigger machine or use the non-amalgamated build.  |
+--------------------------------------------------------------------------+
/Applications/Xcode.app/Contents/Developer/usr/bin/make all "LJCORE_O=ljamalg.o"
gcc     -o host/minilua host/minilua.o -lm   
ld: library not found for -lgcc_s.10.4
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [host/minilua] Error 1
make[1]: *** [amalg] Error 2
make: *** [amalg] Error 2