buildsys: fix c++ compilation

Based on code by: carl
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

common.mak

@@ -28,7 +28,7 @@ CPPFLAGS   := $(IFLAGS) $(CPPFLAGS)
 CFLAGS     += $(ECFLAGS)
 CCFLAGS     = $(CPPFLAGS) $(CFLAGS)
 ASFLAGS    := $(CPPFLAGS) $(ASFLAGS)
-CXXFLAGS   := $(CFLAGS) $(CXXFLAGS)
+CXXFLAGS   += $(CPPFLAGS) $(CFLAGS)
 YASMFLAGS  += $(IFLAGS) -I$(SRC_PATH)/libavutil/x86/ -Pconfig.asm
 HOSTCCFLAGS = $(IFLAGS) $(HOSTCFLAGS)
 LDFLAGS    := $(ALLFFLIBS:%=-Llib%) $(LDFLAGS)

configure

@@ -688,7 +688,7 @@ check_cxx(){
     log check_cxx "$@"
     cat > $TMPCPP
     log_file $TMPCPP
-    check_cmd $cxx $CPPFLAGS $CFLAGS $CXXFLAGS "$@" -c -o $TMPO $TMPCPP
+    check_cmd $cxx $CPPFLAGS $CFLAGS $CXXFLAGS "$@" $CXX_C -o $TMPO $TMPCPP
 }
 
 check_cpp(){
@@ -1977,6 +1977,7 @@ AS_O='-o $@'
 CC_C='-c'
 CC_E='-E -o $@'
 CC_O='-o $@'
+CXX_C='-c'
 CXX_O='-o $@'
 LD_O='-o $@'
 HOSTCC_C='-c'
@@ -3900,6 +3901,7 @@ AS_C=$AS_C
 AS_O=$AS_O
 CC_C=$CC_C
 CC_O=$CC_O
+CXX_C=$CXX_C
 CXX_O=$CXX_O
 LD_O=$LD_O
 LDFLAGS=$LDFLAGS