Explicitly force building in C++98 mode

GCC 6 defaults to C++14 (or more precisely, gnu++14) instead of C++98
(or gnu++98).

The fdk-aac source doesn't support being built in this mode at the
moment, since it relies on narrowing conversion from unsigned 32 bit
integers to FIXP_DBL (which is a signed data type of the same size).

The same approach is used upstream in Android as well, since
d52f3747.

This fixes buliding with GCC 6.

Makefile.am

@@ -12,7 +12,7 @@ AM_CPPFLAGS = \
     -I$(top_srcdir)/libFDK/include \
     -I$(top_srcdir)/libPCMutils/include
 
-AM_CXXFLAGS = -fno-exceptions -fno-rtti
+AM_CXXFLAGS = -fno-exceptions -fno-rtti -std=c++98
 libfdk_aac_la_LINK = $(LINK) $(libfdk_aac_la_LDFLAGS)
 # Mention a dummy pure C file to trigger generation of the $(LINK) variable
 nodist_EXTRA_libfdk_aac_la_SOURCES = dummy.c