Add a simple configure script.

This is *not* based on autoconf. In fact, this doesn't actually
configure anything, (one can compile notmuch directly with just
"make" without running configure if the dependencies are all
satisfied).

The only thing that this configure script does is to check for the
presence of the various dependencies and provide some guidance to
the user if they are not all available.

diff --git a/configure b/configure
new file mode 100755 (executable)
index 0000000..fe46c8e
--- /dev/null
+++ b/configure
@@ -0,0 +1,123 @@
+#! /bin/sh
+
+cat <<EOF
+Welcome to Notmuch, a system for indexing, searching and tagging your email.
+
+We hope that the process of building and installing notmuch is quick
+and smooth so that you can soon be reading and processing your email
+more efficiently than ever.
+
+If anything goes wrong in this process, please do as much as you can
+to figure out what could be different on your machine compared to
+those of the notmuch developers. Then, please email those details to
+Carl Worth <cworth@cworth.org> so that we can hopefully make future
+versions of notmuch easier for you to use.
+
+We'll now investigate your system to find verify that various software
+components that notmuch relies on are available.
+
+EOF
+
+errors=0
+
+if pkg-config --version > /dev/null 2>&1; then
+    have_pkg_config=1
+else
+    have_pkg_config=0
+fi
+
+if xapian-config --version > /dev/null 2>&1; then
+    echo "Checking for Xapian development files... Yes."
+    have_xapian=1
+else
+    echo "Checking for Xapian development files... No."
+    have_xapian=0
+    errors=$((errors + 1))
+fi
+
+if pkg-config --modversion gmime-2.4 > /dev/null 2>&1; then
+    echo "Checking for GMime 2.4 development files... Yes."
+    have_gmime=1
+else
+    echo "Checking for GMime 2.4 development files... No."
+    have_gmime=0
+    errors=$((errors + 1))
+fi
+
+if pkg-config --modversion talloc > /dev/null 2>&1; then
+    echo "Checking for talloc development files... Yes."
+    have_talloc=1
+else
+    echo "Checking for talloc development files... No."
+    have_talloc=0
+    errors=$((errors + 1))
+fi
+
+if [ $errors -gt 0 ]; then
+    cat <<EOF
+
+*** Error: The dependencies of notmuch could not be satisfied. You will
+need to install the following packages before being able to compile
+notmuch:
+
+EOF
+    if [ $have_xapian -eq 0 ]; then
+       echo "  Xapian library (including development files such as headers)"
+       echo "  http://xapian.org/"
+    fi
+    if [ $have_gmime -eq 0 ]; then
+       echo "  GMime 2.4 library (including development files such as headers)"
+       echo "  http://spruce.sourceforge.net/gmime/"
+    fi
+    if [ $have_talloc -eq 0 ]; then
+       echo "  The talloc library (including development files such as headers)"
+       echo "  http://talloc.samba.org/"
+    fi
+    cat <<EOF
+
+On a modern, package-based operating system such as Debian, you can
+install all of the dependencies with the following simple command
+line:
+
+       sudo apt-get install libxapian-dev libgmime-2.4-dev libtalloc-dev
+
+On other systems, a similar command can be used, but the details of the 
+package names may be different, (such as "devel" in place of "dev").
+
+EOF
+    if [ $have_pkg_config -eq 0 ]; then
+cat <<EOF
+Note: the pkg-config program is not available. Both this configure
+script and the Makefile of notmuch use pkg-config to find the
+compilation flags required to link against the various libraries
+needed by notmuch. It's possible you simply need to install pkg-config
+with a command such as:
+
+       sudo apt-get install pkg-config
+
+But if pkg-config is not available for your system, then you will need
+to manually edit the notmuch Makefile to set NOTMUCH_CFLAGS and
+NOTMUCH_LDFLAGS to the correct values without calling pkg-config.
+
+EOF
+    fi
+cat <<EOF
+When you have installed the necessary dependencies, you can run
+configure again to ensure the packages can be found, or simply run
+"make" to compile notmuch.
+
+EOF
+    exit 1
+else
+cat <<EOF
+
+All required packages were found. You may now run the following
+commands to compile and install notmuch:
+
+       make
+       sudo make install
+
+EOF
+    exit 0
+fi
+cat <<EOF