AM_CPPFLAGS is a documented Automake variable for C preprocessor flags
that should not be overridden when compiling the package.
There is no need to have two additional variables that are ultimately
merged into AM_CPPFLAGS.
Their names are also confusing. EXTDA_DIST is a documented Automake
variable. Everything else that starts with "EXTRA" is not.
libdir is for shared and static libraries. Some xrdp libraries are loaded
dynamically from a non-standard directory, so they are used like modules.
Having separate libdir and moduledir would eventually allow to separate
dynamically loaded modules from shared libraries.
AM_CPPFLAGS is for flags passed to the preprocessor, such as defines and
includes. AM_CFLAGS is for flags affecting the compiler, such as debug
and optimization settings.
INCLUDES is an obsolete name. Users can pass INCLUDES and break
compilation. AM_CPPFLAGS is more explicit that the flags come from
Automake and should not be overridden.
