Show error message on when building from git master.

Author: pavoljuhas

src/SConscript

@@ -48,6 +48,25 @@ env['cctbxobjs'] = []
 env['objcrystobjs'] = []
 env['lib_includes'] = []
 
+# Check if we are in makesdist-only master branch
+
+MY_GIT_MASTER_ERROR_MSG = """
+Source files missing.  We may be in the master git branch, which,
+has only scripts for fetching upstream sources.  Checkout one of
+the release tags, e.g.,
+
+    git checkout v1.9.8b
+"""
+skip_source_check = GetOption('clean') or GetOption('help')
+sources_exist = all([File(f).exists() for f in '''
+        ObjCryst/ObjCryst/Crystal.h
+        newmat/newmat.h
+        cctbx/include/cctbx/coordinates.h
+    '''.split()])
+if not (skip_source_check or sources_exist):
+    print MY_GIT_MASTER_ERROR_MSG
+    Exit(1)
+
 # Subsidiary SConscripts -----------------------------------------------------
 
 # These will create the built objects and header file lists.