Unhelpful configure error on OSX for 8u

[David Holmes]

configure:20727: The C compiler (located as /usr/bin/gcc) does not seem 
to be the required GCC compiler.
configure:20729: The result from running with --version was: "Configured 
with: --prefix=/Applications/Xcode.app/Contents/Developer/usr 
--with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk/usr/include/c++/4.2.1"
configure:20731: error: GCC compiler is required. Try setting 
--with-tools-dir.

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Would be nice if it actually stated what the required gcc version is! I 
can't see anything in toolchain.m4

README-builds.html is not very helpful in getting an OSX system setup 
for builds, given the apparent need to downgrade the Xcode tools for 8u :(

David