Other answers to this question aren't helpful for specifically libical, because its configure script is finicky. You need to have a ton of environment variables right. These scripts have figured all them out for libical.
http://code.google.com/p/mwiphonesdk/source/browse/trunk/iMADE/PrepTasks/05+Event+Calendar/Packers+Schedule/libical/build+scripts/
Download the above scripts, and tweak the build_(platform).sh to find the right compilers and SDK folders. This will change depending on what you are targeting and how recent your Xcode developer tools are. Finding the right values should be pretty easy, just look in the same locations for what they are called on your system.
The output will be a fat ".a" files with binaries for the simulator and device.
In the case these files disappear, I've collected the truly important one (build_arm.sh, the cross-compile) below:
#!/bin/sh
# Created by Robert Carlsen on 15.07.2009.
# build an arm / i386 / x64 lib of standard linux project
#
# adopted from: http://latenitesoft.blogspot.com/2008/10/iphone-programming-tips-building-unix.html
#
# copied from: http://robertcarlsen.net/2009/07/15/cross-compiling-for-iphone-dev-884
#
# configured for libical
#
# Note:
# To run with the iPhone the assembly just be a Universal binary (FAT) with i386 arch for the simulator
# and arm arch for the iPhone hardware which has the arm processor.
# set the prefix
PREFIX=${HOME}/Library/libical
OUTPUTDIR=../libical-build
export ARCH=armv6
export GCCARCH=arm-apple-darwin9
export GCCVERSION=4.2.1
# Select the desired iPhone SDK
export DEVROOT=/Developer/Platforms/iPhoneOS.platform/Developer
export SDKROOT=$DEVROOT/SDKs/iPhoneOS3.1.2.sdk
if [ ! -d $DEVROOT ]
then
echo "Developer Root not found! - $DEVROOT"
exit
fi
echo "DEVROOT = $DEVROOT"
if [ ! -d $SDKROOT ]
then
echo "SDK Root not found! - $SDKROOT"
exit
fi
echo "SDKROOT = $SDKROOT"
# Set up relevant environment variables
export CPPFLAGS="-I$SDKROOT/usr/lib/gcc/$GCCARCH/$GCCVERSION/include/ -I$SDKROOT/usr/include/"
export CFLAGS="$CPPFLAGS -pipe -no-cpp-precomp -isysroot $SDKROOT"
export CPP="$DEVROOT/usr/bin/cpp $CPPFLAGS"
export CXXFLAGS="$CFLAGS"
export LDFLAGS="-L$SDKROOT/usr/lib/ "
CC=$DEVROOT/usr/bin/$GCCARCH-gcc-$GCCVERSION
CXX=$DEVROOT/usr/bin/$GCCARCH-g++-$GCCVERSION
HOST=arm-apple-darwin
if [ ! -f $CC ]
then
echo "C Compiler not found! - $CC"
exit
fi
if [ ! -f $CXX ]
then
echo "C++ Compiler not found! - $CXX"
exit
fi
# TODO: add custom flags as necessary for package
./configure --prefix=$PREFIX --disable-dependency-tracking CXX=$CXX CC=$CC LD=$DEVROOT/usr/bin/ld --host=$HOST
make -j4
# copy the files to the arch folder
mkdir -p $OUTPUTDIR
mkdir -p $OUTPUTDIR/$ARCH
cp `find . -name \*.a` $OUTPUTDIR/$ARCH/
lipo -info $OUTPUTDIR/$ARCH/*.a
echo $ARCH DONE
echo "See $OUTPUTDIR"