fix build on OSX when only CLT is installed

Based on a patch by Wael M. Nasreddine.  If Xcode is not installed
but command line tools are, the build will still work.  So detect
when xcode -version fails, and if so just do the build like normal.

Signed-off-by: Bob Copeland <[email protected]>

Makefile

@@ -11,6 +11,10 @@ CFLAGS += -MMD
 
 UNAME_S := $(shell sh -c 'uname -s 2>/dev/null || echo not')
 ifeq ($(UNAME_S),Darwin)
+HAS_XCODE := $(shell sh -c 'xcodebuild -version 2>/dev/null && echo 1')
+endif
+
+ifeq ($(HAS_XCODE),1)
 SDKROOT ?= $(shell xcodebuild -version -sdk macosx | sed -n 's/^Path: \(.*\)/\1/p')
 CFLAGS += -Wno-deprecated-declarations -isysroot $(SDKROOT) -I$(SDKROOT)/usr/include/libxml2
 LDLIBS = -lcurl -lxml2 -lssl -lcrypto