fix PowerPC detection under Darwin

Test if the arch value (`uname -p`) is "powerpc".

PowerPC based systems no longer try to use xcrun to determine where
things like INCDIR (include files) are located.

CHANGES

@@ -27,15 +27,15 @@ The following are the changes from calc version 2.14.2.1 to date:
 	ARCH_CFLAGS+= -arch ppc
 
     Old Apple Power PC systems should be detected by the
-    "uname -m" command returning "ppc".  One may force the
+    "uname -p" command returning "powerpc".  One may force the
     Power PC mode by adding to the end of any make command:
 
-	target=Darwin hardware=ppc
+	make ...make_args... target=Darwin arch=powerpc
 
     or by adding the following in the Makefile.local file:
 
 	target= Darwin
-	hardware= ppc
+	arch= powerpc
 
     Improved the output of the calcinfo rule by adding echos
     of various uname values as well as some top Makefile variables.

Makefile

@@ -841,8 +841,21 @@ HAVE_STRLCAT=
 #
 ifeq ($(target),Darwin)
 
-# default INCDIR for macOS
-INCDIR= $(shell xcrun --show-sdk-path --sdk macosx)/usr/include
+# determine default INCDIR for macOS
+#
+ifeq ($(arch),powerpc)
+#
+# Default location for old systems such as Mac OS X 10.6 Snow Leopard
+#
+INCDIR= /usr/include
+#
+else
+#
+# Modern macOS such as macOS 10.11.6 and later
+#
+INCDIR= $(shell xcrun --sdk macosx --show-sdk-path 2>/dev/null)/usr/include
+#
+endif
 
 else
 #
@@ -1641,7 +1654,7 @@ endif
 #
 #	-std=gnu99 -arch ppc
 #
-ifeq ($(hardware),ppc)
+ifeq ($(arch),powerpc)
 COMMON_CFLAGS+= -std=gnu99
 COMMON_LDFLAGS+= -std=gnu99
 ARCH_CFLAGS+= -arch ppc