configure: pregenerate interface files

Implements: https://todo.sr.ht/~jeremy/esv/8

configure

@@ -5,6 +5,8 @@
 set -e
 
 mkf=config.mk
+di='di'
+imports="$di"
 objs='esvapi.o util.o initial.o'
 srcs='esvapi.d util.d initial.d'
 
@@ -115,6 +117,18 @@ gen_DC
 gen_CFLAGS
 gen_LDFLAGS
 
+for directory in $imports; do
+	mkdir -p directory
+	Iflags="$(printf '%s -I%s' "$Iflags" "$directory" | xargs)"
+done
+
+for src in $srcs; do
+	! (echo "$src" | grep -Eq '\.d$') \
+		&& throw "$src: invalid source file extension"
+	[ -t 2 ] && printf ' (DI) %s        \r' "$src" 1>&2
+	"$dc" -o- -op -H -Hd="$di" "$src"
+done
+
 rm -f "$mkf"
 
 {
@@ -122,7 +136,7 @@ rm -f "$mkf"
 _DC = %s
 _CFLAGS = %s
 _LDFLAGS = %s
-' "$dc" "$cflags" "$ldflags"
+' "$dc" "$(echo "$cflags $Iflags" | xargs)" "$ldflags"
 
 	## generate obj list
 	printf '_OBJS ='