build: put the generated header under include/ for relocated builds

Previously the generated database/database.h was written to the build
top and exposed with -I${MAGICDIR}.  Out-of-tree that placed the raw
build directory on the compiler's include search path.

Introduce ${GENINC}, the single include dir for generated headers:
  * in-tree  (build top == source top): ${MAGICDIR}, so the header stays
    at $(builddir)/database/database.h and -I${GENINC} is exactly the old
    -I${MAGICDIR} -- behaviour is unchanged.
  * relocated (out-of-tree): ${MAGICDIR}/include, so the header is written
    to $(builddir)/include/database/database.h and only that clean
    include/ dir is on the search path; the raw build top never is.

* scripts/defs.mak.in: derive GENINC from BUILD_TOP vs MAGICSRC; add
  DATABASE_H = ${GENINC}/database/database.h; CPPFLAGS uses -I${GENINC}.
* Makefile.in: the makedbh rule targets ${DATABASE_H} (mkdir its dir);
  every prereq and the distclean use ${DATABASE_H}.
* database/Makefile: the fallback delegate targets ${DATABASE_H}.

Verified: relocated `make modules` builds all 33 module libraries with
the header at build/include/database/database.h and no raw build-top on
any -I line; in-tree the header stays at database/database.h and the
-I set is unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

scripts/defs.mak.in

@@ -32,6 +32,20 @@ srcdir                 := $(MAGICSRC)$(MODULE_SUBPATH)
 ifneq ($(MODULE_SUBPATH),)
 VPATH                  := $(srcdir)
 endif
+
+# Generated headers (currently just database/database.h) are exposed through a
+# single include directory, ${GENINC}:
+#   * in-tree  (build top == source top): the top itself, so the header stays at
+#     $(builddir)/database/database.h and -I${GENINC} == the old -I${MAGICDIR}.
+#   * relocated (out-of-tree): a clean $(builddir)/include, so #include
+#     <database/database.h> resolves under include/ and the raw build top is
+#     never on the compiler search path.
+ifeq ($(BUILD_TOP),$(MAGICSRC))
+GENINC                 := ${MAGICDIR}
+else
+GENINC                 := ${MAGICDIR}/include
+endif
+DATABASE_H             := ${GENINC}/database/database.h
 # --------------------------------------------------------------------------
 
 prefix = @prefix@
@@ -141,10 +155,10 @@ CXX		       = @CXX@
 ###   Status: NOT-STABILITY-AFFECTING (if your binary links you are good to continue)
 #FEATURE_FLAGS += -DSUPPORT_REMOVE_MALLOC_LEGACY
 
-# -I${MAGICDIR}: build top, for the generated database/database.h
+# -I${GENINC}:  generated headers (build top in-tree, build/include relocated)
 # -I${MAGICSRC}: source top, for "<module>/<header>.h" includes
 # -I${srcdir}:   this module's source dir, for same-directory headers
-CPPFLAGS               = -I${MAGICDIR} -I${MAGICSRC} -I${srcdir} @CPPFLAGS@
+CPPFLAGS               = -I${GENINC} -I${MAGICSRC} -I${srcdir} @CPPFLAGS@
 CXXFLAGS               = @CXXFLAGS@
 # Bundled GNU readline: its headers use <readline/*.h> includes, so the readline
 # source's parent directory must be on the include path.  Only for the bundled