Defining LICENSE_FILE isn't required if there's a copy under /usr/ports/Templates/Licenses/

Maybe define as USES=localbase

non-free is a poor description, better copy LICENSE* section from audio/fdk-aac and convert to LICENSE_COMB=multi e.g.,

FDK_AAC_DESC=	Enable Fraunhofer FDK AAC Codec

FDK_AAC_VARS=		LICENSE+=FDK_AAC LICENSE_COMB=multi
LICENSE_NAME_FDK_AAC=	Software License for The Fraunhofer FDK AAC Codec Library for Android
LICENSE_FILE_FDK_AAC=	${BUILD_WRKSRC}/contrib/fdkaac/${DISTFILES:Mfdk*:R:R}/NOTICE
LICENSE_PERMS_FDK_AAC=	dist-mirror pkg-mirror auto-accept

According to make/configure.py the option is named --enable-fdk-aac. You can also use _CONFIGURE_ENABLE helper here i.e.,

FDK_AAC_CONFIGURE_ENABLE=	fdk-aac

Do you still need it after USES=localbase ?

To move above .include <bsd.port.options.mk> this can be converted to

# Enforce linking to the system c++ standard lib
USES+=          compiler:features
LDFLAGS+=       -l${COMPILER_FEATURES:Mlib*c++:S/lib//}

Maybe define as post-extract-FDK_AAC-on: if you're already using option helpers. See 20150701 entry in /usr/ports/CHANGES

Ditto, maybe use post-patch-FDK_AAC-on: