It is very difficult to install the ghc-8.6.1 release candidates (both RC1 and RC2) on MacOS because several components attempt to link directly to /usr/local/opt/gmp/lib/libgmp.10.dylib. This is not a standard path.

See the offending LC_LOAD_DYLIB command here:

$ jtool -l libraries/base/dist-install/build/libHSbase-4.12.0.0-ghc8.6.0.20180714.dylib
LC 00: LC_SEGMENT_64          Mem: 0x000000000-0x4e1000 __TEXT
        Mem: 0x0000010b0-0x0004d12ba            __TEXT.__text   (Normal)
        Mem: 0x0004d12ba-0x0004d1686            __TEXT.__stubs  (Symbol Stubs)
        Mem: 0x0004d1688-0x0004d1cee            __TEXT.__stub_helper    (Normal)
        Mem: 0x0004d1cee-0x0004df75b            __TEXT.__cstring        (C-String Literals)
        Mem: 0x0004df760-0x0004e0f40            __TEXT.__const
        Mem: 0x0004e0f40-0x0004e1000            __TEXT.__unwind_info
LC 01: LC_SEGMENT_64          Mem: 0x0004e1000-0x5a7000 __DATA
        Mem: 0x0004e1000-0x0004e30f0            __DATA.__got    (Non-Lazy Symbol Ptrs)
        Mem: 0x0004e30f0-0x0004e3100            __DATA.__nl_symbol_ptr  (Non-Lazy Symbol Ptrs)
        Mem: 0x0004e3100-0x0004e3610            __DATA.__la_symbol_ptr  (Lazy Symbol Ptrs)
        Mem: 0x0004e3610-0x0004e3618            __DATA.__mod_init_func  (Module Init Function Ptrs)
        Mem: 0x0004e3620-0x0004f4bc0            __DATA.__const
        Mem: 0x0004f4bc0-0x0005a6920            __DATA.__data
        Mem: 0x0005a6920-0x0005a6928            __DATA.__bss    (Zero Fill)
LC 02: LC_SEGMENT_64          Mem: 0x0005a7000-0xb8c000 __LINKEDIT
LC 03: LC_ID_DYLIB              @rpath/libHSbase-4.12.0.0-ghc8.6.0.20180714.dylib
LC 04: LC_DYLD_INFO
LC 05: LC_SYMTAB
        Symbol table is at offset 0x6a5348 (6968136), 118090 entries
        String table is at offset 0x873d78 (8863096), 3241616 bytes
LC 06: LC_DYSYMTAB
        81438 local symbols at index     0
        35948 external symbols at index  81438
          704 undefined symbols at index 117386
           No TOC
           No modtab
         1380 Indirect symbols at offset 0x8727e8

LC 07: LC_UUID                  UUID: B4C0C347-131F-317B-BA52-EE23F5C5CABA
LC 08: LC_VERSION_MIN_MACOSX    Minimum OS X version:    10.12.0
LC 09: LC_SOURCE_VERSION        Source Version:          0.0.0.0.0
LC 10: LC_LOAD_DYLIB            /usr/lib/libiconv.2.dylib
LC 11: LC_LOAD_DYLIB            @rpath/libHSinteger-gmp-1.0.2.0-ghc8.6.0.20180714.dylib
LC 12: LC_LOAD_DYLIB            @rpath/libHSghc-prim-0.5.3-ghc8.6.0.20180714.dylib
LC 13: LC_LOAD_DYLIB            /usr/local/opt/gmp/lib/libgmp.10.dylib
LC 14: LC_LOAD_DYLIB            /usr/lib/libSystem.B.dylib
LC 15: LC_RPATH                 @loader_path/../integer-gmp-1.0.2.0
LC 16: LC_RPATH                 @loader_path/../ghc-prim-0.5.3
LC 17: LC_RPATH                 @loader_path/../rts
LC 18: LC_FUNCTION_STARTS       Offset: 6876632, Size: 91504 (0x68edd8-0x6a5348) with 83924 functions
LC 19: LC_DATA_IN_CODE          Offset: 6968136, Size: 0 (0x6a5348-0x6a5348)

The other offenders are libHSbinary and libHSinteger-gmp.

Without changing these load commands, you'll get the following error:

$ ./configure --prefix=...
$ make install
<snip>
dyld: Library not loaded: /usr/local/opt/gmp/lib/libgmp.10.dylib
  Referenced from: ./libraries/base/dist-install/build/libHSbase-4.12.0.0-ghc8.6.0.20180714.dylib
  Reason: image not found
make[1]: *** [install_packages] Abort trap: 6
make: *** [install] Error 2

I tried passing the --with-gmp-libraries option to configure, but that did not help.

I guess this is just a packaging problem, but figured you should be aware before the official release...