configure: add a proper configure script

It's not very good, but it supports all of the options by the
previous far-worse Makefile configure targets, and also now supports
out-of-tree builds.

Also include the relevent updates to the .buildbot script and
.gitignore.

.buildbot

@@ -4,19 +4,25 @@ set -e
 
 do_build()
 {
-    make config-default
+    mkdir build || true
+    cd build
+    ../configure
     make
 }
 
 do_test()
 {
-    make config-debug
+    mkdir build || true
+    cd build
+    ../configure --debug
     make check
 }
 
 do_coverage()
 {
-    make config-coverage
+    mkdir build || true
+    cd build
+    ../configure --coverage
     make coverage-report
     mkdir -p ./buildbot-upload/coverage/
     cp -r ./build/coverage/html/* ./buildbot-upload/coverage/