Support out-of-source builds.

Thanks to Robin Haberkorn.

Makefile

@@ -2,30 +2,33 @@
 
 .SUFFIXES: .cxx .c .o .h .a
 
+srcdir ?= .
+basedir = $(srcdir)/..
+
 AR = ar
 CC = gcc
 CXX = g++
-CXXFLAGS = -std=c++17 -pedantic -DCURSES -DSCI_LEXER -I../include -I../src -Wall
+CXX_BASE_FLAGS = -std=c++17 -pedantic -DCURSES -DSCI_LEXER -I$(basedir)/include -I$(basedir)/src -Wall
 ifdef DEBUG
-  CXXFLAGS += -DDEBUG -g
+  CXX_BASE_FLAGS += -DDEBUG -g
 else
-  CXXFLAGS += -DNDEBUG -Os
+  CXX_BASE_FLAGS += -DNDEBUG -Os
 endif
 CURSES_FLAGS =
 
-scintilla = ../bin/scintilla.a
+scintilla = $(basedir)/bin/scintilla.a
 sci = AutoComplete.o CallTip.o CaseConvert.o CaseFolder.o CellBuffer.o ChangeHistory.o \
   CharacterCategoryMap.o CharacterType.o CharClassify.o ContractionState.o DBCS.o Decoration.o \
   Document.o EditModel.o Editor.o EditView.o Geometry.o Indicator.o KeyMap.o LineMarker.o \
   MarginView.o PerLine.o PositionCache.o RESearch.o RunStyles.o ScintillaBase.o Selection.o \
   Style.o UndoHistory.o UniConversion.o UniqueString.o ViewStyle.o XPM.o
 
-vpath %.h ../src ../include
-vpath %.cxx ../src
+vpath %.h $(srcdir) $(basedir)/src $(basedir)/include
+vpath %.cxx $(srcdir) $(basedir)/src
 
 all: $(scintilla)
 $(sci) PlatCurses.o ScintillaCurses.o: %.o: %.cxx
-	$(CXX) $(CXXFLAGS) $(CURSES_FLAGS) -c $<
+	$(CXX) $(CXX_BASE_FLAGS) $(CXXFLAGS) $(CURSES_FLAGS) -c $<
 $(scintilla): $(sci) PlatCurses.o ScintillaCurses.o
 	$(AR) rc $@ $^
 	touch $@