INSTALL_TARGET=install-strip runs into "permission denied"

[Gerald Pfeifer]

The ports q/a framework has been suggesting this for a while, so 
I added INSTALL_TARGET=install-strip to lang/gcc5/Makefile.

Using install-strip for vanilla GCC builds (from source, outside
the FreeBSD Ports framework) works just fine.

In the context of Ports this runs into a permission problem since
install sets binaries to r-x, at which point strip then fails.

Here is the build log:

install   -m 555 fixinc.sh .../prefix/gcc5/libexec/gcc5/gcc/i386-portbld-freebsd10.1/5.0.0/install-tools/fixinc.sh
install  -s  -m 555 fixincl .../prefix/gcc5/libexec/gcc5/gcc/i386-portbld-freebsd10.1/5.0.0/install-tools/fixincl
install   -m 555 mkheaders .../prefix/gcc5/libexec/gcc5/gcc/i386-portbld-freebsd10.1/5.0.0/install-tools/mkheaders
test -z 'strip' || strip .../prefix/gcc5/libexec/gcc5/gcc/i386-portbld-freebsd10.1/5.0.0/install-tools/fixincl
strip: unable to copy file '.../prefix/gcc5/libexec/gcc5/gcc/i386-portbld-freebsd10.1/5.0.0/install-tools/fixincl'; reason: Permission denied
Makefile:191: recipe for target 'install-strip' failed
gmake[3]: *** [install-strip] Error 1

How do we best go about this?

Gerald