With 
# make -C /usr/ports/ftp/curl -DUSE_C3 showconfig | grep HTT
     HTTP2=off: HTTP protocol version 2.0 support
The curl package will build with http2
# ldd `which curl`
/usr/local/bin/curl:
        libcurl.so.4 => /usr/local/lib/libcurl.so.4 (0x80084f000)
        libz.so.6 => /lib/libz.so.6 (0x800ac7000)
        libthr.so.3 => /lib/libthr.so.3 (0x800ce0000)
        libc.so.7 => /lib/libc.so.7 (0x800f08000)
        libnghttp2.so.14 => /usr/local/lib/libnghttp2.so.14 (0x8012b4000)
        libssh2.so.1 => /usr/local/lib/libssh2.so.1 (0x8014db000)
        libssl.so.43 => /usr/local/lib/libssl.so.43 (0x801706000)
        libcrypto.so.41 => /usr/local/lib/libcrypto.so.41 (0x80195d000)

Commenting out, from the curl/Makefile doesn't help
#HTTP2_CONFIGURE_WITH=  libnghttp2
#HTTP2_LIB_DEPENDS=     libnghttp2.so:www/libnghttp2
#HTTP2_USES=            pkgconfig

so I've just added to CONFIGURE_ARGS, 
--without-nghttp2

which does the trick.  From the diff of the build logs, it looks like pkg-config is being too smart.

< checking for pkg-config... (cached) /usr/local/bin/pkg-config
< checking for libnghttp2 options with pkg-config... found
< configure: -l is -lnghttp2
< configure: -I is -I/usr/local/include
< configure: -L is -L/usr/local/lib
< checking for nghttp2_option_set_no_recv_client_magic in -lnghttp2... yes
< checking nghttp2/nghttp2.h usability... yes
< checking nghttp2/nghttp2.h presence... yes
< checking for nghttp2/nghttp2.h... yes
741c732
< configure: using LIBS: -lnghttp2 -lssh2 -lssh2 -lssl -lcrypto -lz -L/usr/local/lib
---
> configure: using LIBS: -lssh2 -lssh2 -lssl -lcrypto -lz -L/usr/local/lib
827c818
<   HTTP2 support:    enabled (nghttp2)
---
>   HTTP2 support:    disabled (--with-nghttp2)


Yes I do build using portmaster, and I probably already have libnghttp2 for testing via apache.  But I don't want http2 as a dependency by curl as it is very widely used across the environment and a minimal executable is highly desired.  (curl replaces fetch and wget use)