[make] record `SDKROOT` at configure-time to use during `make`

Summary: This can help on Mojave.

Reviewed By: katiejots

Differential Revision: D13956571

fbshipit-source-id: 6dad17166

Makefile.autoconf.in

@@ -66,6 +66,7 @@ PATCHELF = @PATCHELF@
 PATH = @PATH@
 prefix = @prefix@
 PYTHON_lxml = @PYTHON_lxml@
+SDKROOT = @SDKROOT@
 SHASUM = @SHASUM@
 USER_JAVA_HOME = @USER_JAVA_HOME@
 UTOP = @UTOP@
@@ -87,3 +88,4 @@ export CPATH := $(CPATH)
 export LIBRARY_PATH := $(LIBRARY_PATH)
 export CAML_LD_LIBRARY_PATH := $(CAML_LD_LIBRARY_PATH)
 export OPAMROOT := $(OPAMROOT)
+export SDKROOT := $(SDKROOT)

configure.ac

@@ -331,6 +331,8 @@ AS_IF([test "$SED" != "xno"], [
 AC_SUBST([GNU_SED])
 
 AC_CHECK_TOOL([BREW], [brew], [no])
+AC_ARG_VAR([SDKROOT], [path to the OSX platform SDK used by clang])
+AC_SUBST([SDKROOT])
 
 AC_CHECK_TOOL([PATCHELF], [patchelf], [no])