configure: Implement a --help option to document --prefix.

Also document that values for CC, CFLAGS, etc. can be specified
via environment variables.

diff --git a/configure b/configure
index ff8baa5e52696686461d82d5cf9728e1c5448a0b..fa8e142bb786e0450992abe823c6a9ee26bffb9e 100755 (executable)
--- a/configure
+++ b/configure
@@ -1,17 +1,70 @@
 #! /bin/sh
 
+# Set several defaults (optionally specified by the user in
+# environemnt variables)
 CC=${CC:-gcc}
 CXX=${CXX:-g++}
 CFLAGS=${CFLAGS:--O2}
 CXXFLAGS=${CXXFLAGS:-\$(CFLAGS)}
 
-# defaults
+# Set the defaults for values the user can specify with command-line
+# options.
 PREFIX=/usr/local
 
-# option parsing
+usage ()
+{
+    cat <<EOF
+Usage: ./configure [options]...
+
+This script configures notmuch to build on your system.
+
+It verifies that dependencies are available, determines flags needed
+to compile and link against various required libraries, and identifies
+whether various system functions can be used or if locally-provided
+replacements will be built instead.
+
+Finally, it allows you to control various aspects of the build and
+installation process.
+
+First, some common variables can specified via environment variables:
+
+       CC              The C compiler to use
+       CFLAGS          Flags to pass to the C compiler
+       CXX             The C++ compiler to use
+       CXXFLAGS        Flags to pass to the C compiler
+       LDFLAGS         Flags to pass when linking
+
+Each of these values can further be controlled by specifying them
+later on the "make" command line.
+
+Additionally, various options can be specified on the configure
+command line.
+
+       --prefix=PREFIX Install files in PREFIX [$PREFIX]
+
+By default, "make install" will install the resulting program to
+$PREFIX/bin, documentation to $PREFIX/share, etc. You can
+specify an installation prefix other than $PREFIX using
+--prefix, for instance:
+
+       ./configure --prefix=\$HOME
+
+EOF
+}
+
+# Parse command-line options
 for option; do
-    if [ "${option%%=*}" = '--prefix' ] ; then
+    if [ "${option}" = '--help' ] ; then
+       usage
+       exit 0
+    elif [ "${option%%=*}" = '--prefix' ] ; then
        PREFIX="${option#*=}"
+    else
+       echo "Unrecognized option: ${option}."
+       echo "See:"
+       echo "  $0 --help"
+       echo ""
+       exit 1
     fi
 done