Clarify that the htscodecs submodule is part of htslib

The original message printed when the submodule files are not
present could be confusing if HTSlib was being built as an
embedded part of SAMtools.  Alter the message slightly to make
it clearer that the submodule needs to be updated in the htslib
checkout.

Fixes samtools/samtools#1364

Author: daviesrob

Makefile

@@ -422,10 +422,10 @@ htscodecs/htscodecs/%.h: | htscodecs/htscodecs
 	;
 htscodecs/htscodecs:
 	@if test -e .git ; then \
-	printf "\\n\\nError: htscodecs submodule files not present.\\n\
+	printf "\\n\\nError: htscodecs submodule files not present for htslib.\\n\
 	Try running: \\n\
 	    git submodule update --init --recursive\\n\
-	and then re-run make.\\n\\n\\n" ; \
+	in the top-level htslib directory and then re-run make.\\n\\n\\n" ; \
 	else \
 	  printf "\\n\\nError: htscodecs submodule files not present and this is not a git checkout.\\n\
 	  You have an incomplete distribution.  Please try downloading one of the\\n\

configure.ac

@@ -319,7 +319,7 @@ included as a submodule.  Try running:
 
   git submodule update --init --recursive
 
-to update it, and then re-run configure.
+in  the top-level htslib directory to update it, and then re-run configure.
 ])],
         [MSG_ERROR([htscodecs submodule files not present.