checking pkg-config is at least version 0.9.0... yes
checking for XFCE4_PANEL... configure: error: Package requirements (xfce4-panel-1.0 >= 3.99.2) were not met:

Package xfce4-panel-1.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `xfce4-panel-1.0.pc'
to the PKG_CONFIG_PATH environment variable
No package 'xfce4-panel-1.0' found

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

Alternatively, you may set the environment variables XFCE4_PANEL_CFLAGS
and XFCE4_PANEL_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.