configure for gcc 6.4.0 build on Fedora 27
Thu Jun 14 06:53:00 GMT 2018

I downloaded the subversion tag for gcc 6.4.0. Within this I used the "contrib/download_prerequisites" script, and thus there now exists subdirectories to the gcc source of:
I also built and installed these in "/usr/local/mpfr-2.4.2/", "/usr/local/gmp-4.3.2/", "/usr/local/mpc-0.8.1/", and "/usr/local/isl-0.15/".
If I use "./configure" for gcc in any way it fails claiming incorrect versions shortly after declaring they are correct. For example, directly from the download_prerequisites content:
./configure --prefix=/usr/local/gcc_6-4-0 --with-mpfr=/usr/local/src/gcc-x86_64/gcc_6_4_0/mpfr-2.4.2/ --with-gmp=/usr/local/src/gcc-x86_64/gcc_6_4_0/gmp-4.3.2 --with-mpc=/usr/local/src/gcc-x86_64/gcc_6_4_0/mpc-0.8.1 --with-isl=/usr/local/src/gcc-x86_64/gcc_6_4_0/isl-0.15
It also fails if I configure for the "/usr/local/" installs:
./gcc_6_4_0/configure --prefix=/usr/local/gcc_6-4-0 --with-mpfr=/usr/local/mpfr-2.4.2/ --with-gmp=/usr/local/gmp-4.3.2 --with-mpc=/usr/local/mpc-0.8.1 --with-isl=/usr/local/isl-0.15
In all cases the log shows the header files were correct, and that the libraries exist, but that for some reason (I suspect a bug) the versions are not accepted:
checking for the correct version of gmp.h... yeschecking for the correct version of mpfr.h... yeschecking for the correct version of mpc.h... yeschecking for the correct version of the gmp/mpfr/mpc libraries... noconfigure: error: Building GCC requires GMP 4.2+, MPFR 2.4.0+ and MPC 0.8.0+.Try the --with-gmp, --with-mpfr and/or --with-mpc options to specifytheir locations.  Source code for these libraries can be found at 
I have tried with the separate objdir as well as in the source tree. Is there a bug preventing the correct versions from being detected? Is there something missing in the documentation needed as a workaround when using the "download_prerequisites" script?

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