meson: rename incbase to incconfig

Makes it clearer what it's for: config.h. See #4.

meson.build

diff --git a/meson.build b/meson.build
index 50e684f..6d5257a 100644
--- a/meson.build
+++ b/meson.build
@@ -55,7 +55,7 @@
 deps = []
 
 conf = configuration_data()
-incbase = include_directories('.')
+incconfig = include_directories('.')
 cpp_args = ['-DHAVE_CONFIG_H']
 
 warn_cflags = [