Check for finding an SDK (the compiler error message is cryptic when …

…this

happens).  Inform of the git repositories of SDKs as a source for them.

build.macos32x86/HowToBuild

@@ -173,8 +173,12 @@ SDKs and Problems
 -----------------
 The makefiles will use the oldest available SDK that is installed that is new
 enough to build the system.  The 32-bit build requires at least the 10.9 SDK.
-To obtain SDKs it is best to web search for "obtaining older sdks for Xcode",
-and follow the instructions you find.
+SDKs are installed in
+	/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/
+To obtain SDKs one way is to web search for "obtaining older sdks for Xcode",
+and follow the instructions you find.  But a more convenient place, at time of
+writing, is
+	git clone https://github.com/phracker/MacOSX-SDKs.git
 
 On Sierra people have reported that the support libraries for the SSLPlugin
 (libssl.dylib and libcrypto.dylib) are missing.  The symptom is an error
@@ -254,5 +258,6 @@ e.g. if in the image directory you might do
 The VM includes many functions intended for VM debugging.  See e.g. functions
 beginning with print in the main VM source (e.g. spursrc/vm/gcc3x-cointerp.c).
 
+
 Eliot Miranda
-August 2017
+April 2018

build.macos32x86/common/Makefile.flags

@@ -9,6 +9,9 @@ SDKsDIR:=$(XCODE)/Platforms/MacOSX.platform/Developer/SDKs
 #Build the oldest SDK installed
 SDKs:=MacOSX10.9.sdk MacOSX10.10.sdk MacOSX10.11.sdk MacOSX10.12.sdk MacOSX10.13.sdk
 SDK:=$(firstword $(realpath $(addprefix $(SDKsDIR)/, $(SDKs))))
+ifeq ($(strip $(SDK)),)
+$(error could not find a valid SDK)
+endif
 TARGET_ARCH:=i386
 TARGET_VERSION_MIN:=10.7
 

build.macos64x64/HowToBuild

@@ -173,8 +173,12 @@ SDKs and Problems
 -----------------
 The makefiles will use the oldest available SDK that is installed that is new
 enough to build the system.  The 64-bit build requires at least the 10.10 SDK.
-To obtain SDKs it is best to web search for "obtaining older sdks for Xcode",
-and follow the instructions you find.
+SDKs are installed in
+	/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/
+To obtain SDKs one way is to web search for "obtaining older sdks for Xcode",
+and follow the instructions you find.  But a more convenient place, at time of
+writing, is
+	git clone https://github.com/phracker/MacOSX-SDKs.git
 
 On Sierra people have reported that the support libraries for the SSLPlugin
 (libssl.dylib and libcrypto.dylib) are missing.  The symptom is an error
@@ -255,4 +259,4 @@ The VM includes many functions intended for VM debugging.  See e.g. functions
 beginning with print in the main VM source (e.g. spursrc/vm/gcc3x-cointerp.c).
 
 Eliot Miranda
-August 2017
+April 2018

build.macos64x64/common/Makefile.flags

@@ -9,6 +9,9 @@ SDKsDIR:=$(XCODE)/Platforms/MacOSX.platform/Developer/SDKs
 #Build the oldest SDK installed (must be MacOSX10.10.sdk and later)
 SDKs:=MacOSX10.10.sdk MacOSX10.11.sdk MacOSX10.12.sdk MacOSX10.13.sdk
 SDK:=$(firstword $(realpath $(addprefix $(SDKsDIR)/, $(SDKs))))
+ifeq ($(strip $(SDK)),)
+$(error could not find a valid SDK)
+endif
 TARGET_ARCH:=x86_64
 TARGET_VERSION_MIN:=10.7