Move "config" test programs to "compat".

It makes sense to me to have the little tests for functionality right
next to the comptability implementations of that same functionality.

But also, this means I can now tab-complete ./configure from the three
initial characters (rather than the seven required previously).

compat/README

@@ -0,0 +1,16 @@
+notmuch/comapt
+
+This directory consists of two things:
+
+1. Small programs used by the notmuch configure script to test for the
+   availability of certain system features, (library functions, etc.).
+
+   For example: have_getline.c
+
+2. Compatibility implementations of those system features for systems
+   that don't provide their own versions.
+
+   For example: getline.c
+
+   The compilation of these files is made conditional on the output of
+   the test programs from [1].

config/README

@@ -1,5 +0,0 @@
-notmuch/config
-
-This directory consists of small programs used by the notmuch
-configure script to test for the availability of certain system
-features, (library functions, etc.).

configure

@@ -250,7 +250,7 @@ EOF
 fi
 
 printf "Checking for getline... "
-if ${CC} -o config/have_getline config/have_getline.c > /dev/null 2>&1
+if ${CC} -o compat/have_getline compat/have_getline.c > /dev/null 2>&1
 then
     printf "Yes.\n"
     have_getline=1
@@ -258,7 +258,7 @@ else
     printf "No (will use our own instead).\n"
     have_getline=0
 fi
-rm -f config/have_getline
+rm -f compat/have_getline
 
 cat <<EOF