Versions that require OCaml < 4.06 fail to build

[spameier]

The following error appears when building Liquidsoap 1.3.2 with OCaml 4.05.0:

[ERROR] The compilation of liquidsoap failed at "/usr/bin/make".                                                                        
                                  
#=== ERROR while compiling liquidsoap.1.3.2 ===================================#                                                        
# context              2.0.5 | linux/x86_64 | ocaml-base-compiler.4.05.0 | https://opam.ocaml.org#bc6d04e5                              
# path                 ~/.opam/4.05.0/.opam-switch/build/liquidsoap.1.3.2
# command              /usr/bin/make                                                                                                    
# exit-code            2                                                                                                                
# env-file             ~/.opam/log/liquidsoap-5084-cb3843.env                                                                           
# output-file          ~/.opam/log/liquidsoap-5084-cb3843.out                                                                           
### output ###                                                                                                                          
# [...]                                                                                                                                 
# OCAMLOPT -o liquidsoap                                                                                                                
# /usr/lib/gcc/x86_64-alpine-linux-musl/9.2.0/../../../../x86_64-alpine-linux-musl/bin/ld: /home/liquidsoap/.opam/4.05.0/lib/ocaml/libasmrun.a(major_gc.o):/home/liquidsoap/.opam/4.05.0/.opam-switch/build/ocaml-base-compiler.4.05.0/asmrun/../byterun/caml/major_gc.h:67: multiple definition of `caml_major_ring'; /home/liquidsoap/.opam/4.05.0/lib/taglib/libtaglib_stubs.a(taglib_stubs.o):(.bss+0x0):[...]      
# /usr/lib/gcc/x86_64-alpine-linux-musl/9.2.0/../../../../x86_64-alpine-linux-musl/bin/ld: /home/liquidsoap/.opam/4.05.0/lib/ocaml/libasmrun.a(major_gc.o):/home/liquidsoap/.opam/4.05.0/.opam-switch/build/ocaml-base-compiler.4.05.0/asmrun/../byterun/caml/major_gc.h:68: multiple definition of `caml_major_ring_index'; /home/liquidsoap/.opam/4.05.0/lib/taglib/libtaglib_stubs.a(taglib_stubs.o):(.bss[...]      
# /usr/lib/gcc/x86_64-alpine-linux-musl/9.2.0/../../../../x86_64-alpine-linux-musl/bin/ld: /home/liquidsoap/.opam/4.05.0/lib/ocaml/libasmrun.a(major_gc.o):/home/liquidsoap/.opam/4.05.0/.opam-switch/build/ocaml-base-compiler.4.05.0/asmrun/../byterun/caml/major_gc.h:69: multiple definition of `caml_major_work_credit'; /home/liquidsoap/.opam/4.05.0/lib/taglib/libtaglib_stubs.a(taglib_stubs.o):(.bs[...]      
# collect2: error: ld returned 1 exit status                                                                                            
# File "caml_startup", line 1:                                                                                                          
# Error: Error during linking                                       
# make[2]: *** [../Makefile.rules:110: liquidsoap] Error 2                                                                              
# make[2]: Leaving directory '/home/liquidsoap/.opam/4.05.0/.opam-switch/build/liquidsoap.1.3.2/src'                                    
# make[1]: *** [../Makefile.rules:67: all-auto-ocaml-prog] Error 2                                                                      
# make[1]: Leaving directory '/home/liquidsoap/.opam/4.05.0/.opam-switch/build/liquidsoap.1.3.2/src'
# make: *** [Makefile.rules:27: all-subdirs] Error 2