I have installed sylpheed-claws 2.0.0 on pdaXrom and am trying to compile a plugin for sylpheed-claws now (the mailmbox 1.10 plugin).
When I ./configure I get:

checking for pkg-config... /opt/native/arm/3.4.6-xscale-softvfp/bin/pkg-config
checking pkg-config is at least version 0.9.0... yes
checking for SYLPHEED_CLAWS... Package sylpheed-claws was not found in the pkg-config search path.
Perhaps you should add the directory containing `sylpheed-claws.pc'
to the PKG_CONFIG_PATH environment variable
No package 'sylpheed-claws' found
configure: error: Package requirements (sylpheed-claws >= were not met:

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables SYLPHEED_CLAWS_CFLAGS
and SYLPHEED_CLAWS_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.

pkg-config does not have an entry for sylpheed-claws.
How can I know which cflags have been used when that program has been compiled, i.e. what to pass to ./configure --SYLPHEED_CLAWS_CFLAGS and --SYLPHEED_CLAWS_LIBS?

Or are there "safe" standard cflag and linker flag values I can pass without knowing the actual ones?