[Issue 18433] rdmd doesn't respect DFLAGS for its cache hash

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18433

--- Comment #2 from Seb <greensunny12 at gmail.com> ---
Argh, apparently this hasn't been fixed in master and DFLAGS is only looked at
when -conf= is set:

```
> DFLAGS="-version=Foo" ../dmd/generated/linux/release/64/dmd -v foo.d && ./foo
predefs   DigitalMars Posix linux ELFv1 LittleEndian D_Version2 all D_SIMD
D_InlineAsm_X86_64 X86_64 CRuntime_Glibc D_LP64 D_PIC assert D_HardFloat
binary    ../dmd/generated/linux/release/64/dmd
version   v2.079.0-284-g23b2e2e0d
config    ../dmd/generated/linux/release/64/dmd.conf
DFLAGS    -I../dmd/generated/linux/release/64/../../../../../druntime/import
-I../dmd/generated/linux/release/64/../../../../../phobos
-L-L../dmd/generated/linux/release/64/../../../../../phobos/generated/linux/release/64
-L--export-dynamic -fPIC
```


```
> DFLAGS="-version=Foo" dmd -conf= -I~/dlang/phobos -I~/dlang/druntime/import -c -v foo.d
predefs   Foo DigitalMars Posix linux ELFv1 LittleEndian D_Version2 all D_SIMD
D_InlineAsm_X86_64 X86_64 CRuntime_Glibc D_LP64 D_PIC assert D_HardFloat
binary    /home/seb/dlang/dmd/generated/linux/release/64/dmd
version   v2.079.0-284-g23b2e2e0d
config    
DFLAGS    -version=Foo
```

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