Let environment provide default values for CPPFLAGS, CPPFLAGS, and LDFLAGS

https://sourceforge.net/p/espeak/patches/19/

---
 src/Makefile |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/src/Makefile
+++ b/src/Makefile
@@ -15,13 +15,13 @@ WX_LIBS = -pthread   `wx-config --libs`
 LIBS=-lstdc++ -lportaudio -lsonic -lm
 #LIBS=-lstdc++ /usr/lib/x86_64-linux-gnu/libportaudio.so.2
 
-CPPFLAGS =  -Wall -g -fexceptions `wx-config --cflags` \
-	-I/usr/include/wx-2.8 \
+CPPFLAGS ?=
+CPPFLAGS += -I/usr/include/wx-2.8 \
 	-DGTK_NO_CHECK_CASTS -D__WXGTK__ -D_FILE_OFFSET_BITS=64 -D_LARGE_FILES \
 	-D_LARGEFILE_SOURCE=1 -DNO_GCC_PRAGMA -D_ESPEAKEDIT
 
-CXXFLAGS = -O2  -Wall -fexceptions `wx-config --cflags` \
-   -DGTK_NO_CHECK_CASTS -D__WXGTK__ -D_FILE_OFFSET_BITS=64 -D_LARGE_FILES -D_LARGEFILE_SOURCE=1 -DNO_GCC_PRAGMA -D_ESPEAKEDIT
+CXXFLAGS ?= -O2 -g -Wall
+CXXFLAGS += -fexceptions `wx-config --cflags`
 
 DEB_HOST_MULTIARCH ?=$(shell dpkg-architecture -qDEB_HOST_MULTIARCH)
 CPPFLAGS += -DNDEBUG
@@ -30,10 +30,10 @@ CXXFLAGS += -DDEB_HOST_MULTIARCH='"$(DEB
 all: espeakedit
 
 .cpp.o:
-	$(CXX) $(CXXFLAGS) -I. -c -fno-exceptions $<
+	$(CXX) $(CPPFLAGS) $(CXXFLAGS) -I. -c -fno-exceptions $<
 
 espeakedit: $(OBJS)
-	$(CXX) -o $@ $(OBJS) $(LIBS) $(WX_LIBS)
+	$(CXX) -o $@ $(OBJS) $(LIBS) $(WX_LIBS) $(LDFLAGS)
 
 clean:
 	rm -f *.o *~
