diff --git a/config/config.mk.in b/config/config.mk.in
index 32f77e81..a911c9d1 100644
--- a/config/config.mk.in
+++ b/config/config.mk.in
@@ -48,14 +48,13 @@ CXX = @CXX@
 GXX = @GXX@
 
 # Common compiler flags (warnings, symbols [-ggdb], optimization [-O2], etc)
+CXXFLAGS = @CXXFLAGS@
 ifeq ($(GXX),yes)
-	CXXFLAGS = -Wall -O2
-else
-	CXXFLAGS = @CXXFLAGS@
+	CXXFLAGS += -Wall
 endif
 
 # Command to run only the preprocessor
-CPP = $(CXX) -E -dD
+CXXCPP = @CXXCPP@
 
 # Preprocessor flags
 CPPFLAGS = @CPPFLAGS@
@@ -71,10 +70,9 @@ EXEEXT := @EXEEXT@
 CC = @CC@
 GCC = @GCC@
 
+CFLAGS = @CFLAGS@
 ifeq ($(GCC),yes)
-	CFLAGS := -Wall -O2
-else
-	CFLAGS := @CFLAGS@
+	CFLAGS += -Wall
 endif
 
 # **********************************************************************
diff --git a/config/configure.ac b/config/configure.ac
index ff082be0..5e79ad00 100644
--- a/config/configure.ac
+++ b/config/configure.ac
@@ -9,6 +9,7 @@ AC_CONFIG_AUX_DIR([./config])
 
 # Checks for programs.
 AC_PROG_CXX
+AC_PROG_CXXCPP
 AC_PROG_CC
 AC_PROG_CPP
 AC_PROG_INSTALL
diff --git a/src/Makefile b/src/Makefile
index 577e0930..6f752137 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -161,7 +161,7 @@ $(sort $(BINOBJ) $(EXIV2OBJ) $(MCOBJ) path-test.o): %.o: %.cpp
 
 %.ii: %.cpp
 	set -e; \
-	$(CPP) $(CPPFLAGS) $(CXXDEFS) $(CXXINCS) $< | sed '/^[ 	]*$$/d' > $@
+	$(CXXCPP) $(CPPFLAGS) $(CXXDEFS) $(CXXINCS) $< | sed '/^[ 	]*$$/d' > $@
 
 # ******************************************************************************
 # Targets