Please describe the problem:
MacPorts intentionally installs all of its software in a unique install path to avoid conflicts between MacPorts built ports and system installed software.  By default this is /opt/local but can be any prefix specified at MacPorts installation.  When building port gimp-gap MacPorts passes this path using the standard --prefix argument to configure.  For ffmpeg to build successfully this
flag as well as appropriate --extra-cflags and --extra-ldflags needs to be passed to ffmpeg's configure.

Steps to reproduce:
Attempt to configure and build gimp-gap by calling

configure --prefix=${prefix} --extra-cflags=-I${prefix}/include --extra-ldflags=-L${prefix}/lib

where ${prefix} is replaced with the MacPorts install prefix (typically /opt/local).

Actual results:
The embedded ffmpeg fails to configure and build because the above configure arguments are not passed to ffmpeg's configure.  Instead a fixed set of arguments contained in extern_libs/configure_options_ffmpeg.txt is used.

Expected results:
gimp-gaps build script should pass any arguments passed to its configure script to ffmpeg's configure script.

Does this happen every time?
yes

Other information:
MacPorts deals with this by editing extern_libs/configure_options_ffmpeg.txt on
the fly before configuration of gimp-gap using the following code in its (tcl) Portfile.  

pre-configure {
    reinplace "s|--enable-shared|--prefix=${prefix} --extra-cflags=-I${prefix}/include --extra-ldflags=-L${prefix}/lib --enable-shared|" \
       ${worksrcpath}/extern_libs/configure_options_ffmpeg.txt
}

here

${prefix} is replaced with the MacPorts install prefix for this specific instance of MacPorts

${worksrcpath} is replaced with the path to the top level of the gimp-gap
distributions's build directory. For the default installation prefix

--enable-shared 

is replaced with

--prefix=/opt/local --extra-cflags=-I$/opt/local/include --extra-ldflags=-L$/opt/local/lib --enable-shared