Disallow GCC 7 builds

If the script detects GCC 7.0.0, the installation script will
download and install 6.1.0 due to incompatilibies between the
library and the current trunk.

Author: Damian Rouson

install.sh-usage

@@ -1,4 +1,4 @@
-  -b --install-branch [arg]           Install the specified repository development branch.
+  -b --install-branch [arg]   Install the specified repository development branch.
   -B --list-branches [arg]    List the available branches in the specified package's repository.
   -c --with-c [arg]           Use specified C compiler. 
   -C --with-cxx [arg]         Use specified C++ compiler. 

prerequisites/acceptable_compiler.f90

@@ -35,5 +35,5 @@
 program main
   use iso_fortran_env, only : compiler_version
   implicit none
-  print *,compiler_version() >= "GCC version 6.1.0 "
+  print *,(compiler_version() >= "GCC version 6.1.0 ") .and. (compiler_version() < "GCC version 7.0.0 ")
 end program

prerequisites/install-functions/find_or_install.sh

@@ -163,7 +163,7 @@ find_or_install()
 
         else
 
-          echo -e "$this_script: Checking whether $executable in PATH wraps gfortran version $(./build.sh -V gcc) or later... "
+          echo -e "$this_script: Checking whether $executable in PATH wraps gfortran version >= $(./build.sh -V gcc) and < 7.0.0 ... "
           $executable acceptable_compiler.f90 -o acceptable_compiler
           $executable print_true.f90 -o print_true
           acceptable=$(./acceptable_compiler)