[PETSc] Require GCC 9 for libgfortran5

The linker that comes with GCC 8 emits a warning on `{i686,x86_64}-linux-musl` and that makes configuration script fail.

P/PETSc/build_tarballs.jl

@@ -14,7 +14,6 @@ sources = [
 # Bash recipe for building across all platforms
 script = raw"""
 cd $WORKSPACE/srcdir/petsc*
-includedir="${prefix}/include"
 atomic_patch -p1 $WORKSPACE/srcdir/patches/petsc_name_mangle.patch
 
 BLAS_LAPACK_LIB="${libdir}/libopenblas.${dlext}"
@@ -88,4 +87,4 @@ dependencies = [
 ]
 
 # Build the tarballs.
-build_tarballs(ARGS, name, version, sources, script, platforms, products, dependencies; preferred_gcc_version = v"6")
+build_tarballs(ARGS, name, version, sources, script, platforms, products, dependencies; preferred_gcc_version = v"9")