fix macOS CI for fpm-bootstrap executables built with gcc-9 (#861)

create symbolic links to the gcc-9 library paths hardcoded in the bootstrapping fpm executables

Author: perazz

.github/workflows/CI.yml

@@ -51,6 +51,17 @@ jobs:
         ln -s /usr/local/bin/gfortran-${GCC_V} /usr/local/bin/gfortran
         which gfortran-${GCC_V}
         which gfortran
+        # Backport gfortran shared libraries to version 9 folder. This is necessary because all macOS releases of fpm 
+        # have these paths hardcoded in the executable (no PIC?). As the gcc ABIs have not changed from 9 to 10, we 
+        # can just create symbolic links for now. This can be removed when an updated fpm release is built with gcc-10  
+        mkdir /usr/local/opt/gcc@9
+        mkdir /usr/local/opt/gcc@9/lib
+        mkdir /usr/local/opt/gcc@9/lib/gcc
+        mkdir /usr/local/opt/gcc@9/lib/gcc/9
+        mkdir /usr/local/lib/gcc/9
+        ln -fs /usr/local/opt/gcc@${GCC_V}/lib/gcc/${GCC_V}/libquadmath.0.dylib /usr/local/opt/gcc@9/lib/gcc/9/libquadmath.0.dylib 
+        ln -fs /usr/local/opt/gcc@${GCC_V}/lib/gcc/${GCC_V}/libgfortran.5.dylib /usr/local/opt/gcc@9/lib/gcc/9/libgfortran.5.dylib
+        ln -fs /usr/local/lib/gcc/${GCC_V}/libgcc_s.1.dylib /usr/local/lib/gcc/9/libgcc_s.1.dylib
 
     - name: Install GFortran Linux
       if: contains(matrix.os, 'ubuntu')