diff --git a/configure b/configure
index 43ae8c8..0686941 100755
*** a/configure
--- b/configure
*************** ac_includes_default="\
*** 627,632 ****
--- 627,633 ----
  
  ac_subst_vars='LTLIBOBJS
  vpath_build
+ PG_SYSROOT
  PG_VERSION_NUM
  PROVE
  FOP
*************** _ACEOF
*** 18815,18820 ****
--- 18816,18830 ----
  
  
  
+ # If we are inserting PG_SYSROOT into CPPFLAGS, do so symbolically not
+ # literally, so that it's possible to override it at build time using
+ # a command like "make ... PG_SYSROOT=path".  This has to be done after
+ # we've finished all configure checks that depend on CPPFLAGS.
+ if test x"$PG_SYSROOT" != x; then
+   CPPFLAGS=`echo "$CPPFLAGS" | sed -e "s| $PG_SYSROOT | \\\$(PG_SYSROOT) |"`
+ fi
+ 
+ 
  
  # Begin output steps
  
diff --git a/configure.in b/configure.in
index 519ecd5..7586deb 100644
*** a/configure.in
--- b/configure.in
*************** $AWK '{printf "%d%04d", $1, $2}'`"]
*** 2357,2362 ****
--- 2357,2371 ----
  AC_DEFINE_UNQUOTED(PG_VERSION_NUM, $PG_VERSION_NUM, [PostgreSQL version as a number])
  AC_SUBST(PG_VERSION_NUM)
  
+ # If we are inserting PG_SYSROOT into CPPFLAGS, do so symbolically not
+ # literally, so that it's possible to override it at build time using
+ # a command like "make ... PG_SYSROOT=path".  This has to be done after
+ # we've finished all configure checks that depend on CPPFLAGS.
+ if test x"$PG_SYSROOT" != x; then
+   CPPFLAGS=`echo "$CPPFLAGS" | sed -e "s| $PG_SYSROOT | \\\$(PG_SYSROOT) |"`
+ fi
+ AC_SUBST(PG_SYSROOT)
+ 
  
  # Begin output steps
  
diff --git a/src/Makefile.global.in b/src/Makefile.global.in
index bdf394b..218c65a 100644
*** a/src/Makefile.global.in
--- b/src/Makefile.global.in
*************** BITCODE_CXXFLAGS = @BITCODE_CXXFLAGS@
*** 241,246 ****
--- 241,247 ----
  
  CPP = @CPP@
  CPPFLAGS = @CPPFLAGS@
+ PG_SYSROOT = @PG_SYSROOT@
  
  override CPPFLAGS := $(ICU_CFLAGS) $(CPPFLAGS)
  
diff --git a/src/template/darwin b/src/template/darwin
index 159d8bb..c05adca 100644
*** a/src/template/darwin
--- b/src/template/darwin
***************
*** 3,16 ****
  # Note: Darwin is the original code name for macOS, also known as OS X.
  # We still use "darwin" as the port name, partly because config.guess does.
  
! # Some configure tests require explicit knowledge of where the Xcode "sysroot"
! # is.  We try to avoid having this leak into configure's results, though.
  if test x"$PG_SYSROOT" = x"" ; then
    PG_SYSROOT=`xcodebuild -version -sdk macosx Path 2>/dev/null`
  fi
  # Old xcodebuild versions may produce garbage, so validate the result.
  if test x"$PG_SYSROOT" != x"" ; then
!   if test \! -d "$PG_SYSROOT" ; then
      PG_SYSROOT=""
    fi
  fi
--- 3,17 ----
  # Note: Darwin is the original code name for macOS, also known as OS X.
  # We still use "darwin" as the port name, partly because config.guess does.
  
! # Select where system include files should be sought.
  if test x"$PG_SYSROOT" = x"" ; then
    PG_SYSROOT=`xcodebuild -version -sdk macosx Path 2>/dev/null`
  fi
  # Old xcodebuild versions may produce garbage, so validate the result.
  if test x"$PG_SYSROOT" != x"" ; then
!   if test -d "$PG_SYSROOT" ; then
!     CPPFLAGS="-isysroot $PG_SYSROOT $CPPFLAGS"
!   else
      PG_SYSROOT=""
    fi
  fi