Review RPATH in *link section of specs file for gcc-7

[drbenmorgan]

Full reproduction details are supplied in SuperNEMO-DBD/brew#9

The core issue is to review these lines in the gcc@7 formula:

homebrew-core/Formula/[email protected]

Lines 221 to 225 in e458dbb

	*link_libgcc:
	#{glibc_installed ? "-nostdlib -L#{libgcc}" : "+"} -L#{libdir} -L#{HOMEBREW_PREFIX}/lib
	*link:
	+ --dynamic-linker #{HOMEBREW_PREFIX}/lib/ld.so -rpath #{libdir} -rpath #{HOMEBREW_PREFIX}/lib

It appears to be the last -rpath entry that causes the issue, overriding any local build setting of the RPATH as it appears before any of the settings for the build. Hence, Home-brew installed libs always override local libs.

A brief test with a local specs file suggests that removing the last -rpath entry resolves that issue, but introduces new errors because some libraries are then not found.

To be investigated further.