configure.ac: fix '--disable-dwarf-debug'

Before the change
    ./configure --disable-dwarf-debug
enabled DWARF debugging unconditionally.

This happened due to use of 5-argument form of `AC_ARG_ENABLE`
without actually checking the passed  `$enableval` parameter:

```
AC_ARG_ENABLE(dwarf-unwind,
    [AC_HELP_STRING([--enable-dwarf-unwind],
        [Enable DWARF unwinding support in the runtime system via elfutils' libdw [default=no]])],
    [AC_CHECK_LIB(dw, dwfl_attach_state,
      [UseLibdw=YES],
      [AC_MSG_ERROR([Cannot find system libdw (required by --enable-dwarf-unwind)])])]
    [UseLibdw=NO]
)
```

Note:

- `[UseLibdw=NO]` is called when `--{enable,disable}-dwarf-unwind`
  is not passed at all as a parameter (ok).
- `[AC_CHECK_LIB(dw, dwfl_attach_state, [UseLibdw=YES],` is called
  for both:

  * `--enable-dwarf-unwind` being passed: `$enableval = "yes"` (ok).
  *  --disable-dwarf-unwind` being passed: `$enableval = "no"` (bad).

The change is to use 3-argument `AC_ARG_ENABLE` and check for passed
value as `"$enable_dwarf_unwind" = "yes"`.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
22 jobs for !1586 with configure-dwarf in 553 minutes and 40 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Lint
passed #143523
lint
ghc-linters

00:01:09

passed #143524
lint
lint-linters

00:01:14

passed #143527
lint
lint-submods-mr

00:00:16

passed #143525
lint
lint-testsuite

00:00:15

passed #143526
lint
typecheck-testsuite

00:00:14

 
  Build
passed #143529
x86_64-linux
hadrian-ghc-in-ghci

00:25:00

passed #143531
x86_64-linux
release-x86_64-linux-deb9-dwarf

01:57:03

passed #145463
x86_64-linux
validate-x86_64-linux-deb9-debug

01:33:14

passed #143528
x86_64-linux
validate-x86_64-linux-deb9-hadrian

03:59:01

failed #143530
x86_64-linux
validate-x86_64-linux-deb9-debug

01:23:29

 
  Full Build
failed #143533
aarch64-linux allowed to fail
validate-aarch64-linux-deb9

03:05:35

passed #143534
x86_64-linux
validate-i386-linux-deb9

01:48:03

passed #143532
x86_64-darwin
validate-x86_64-darwin

01:32:13

passed #143535
x86_64-linux
validate-x86_64-linux-deb9-integer-simple

01:13:07

passed #143536
x86_64-linux
validate-x86_64-linux-deb9-unreg

03:02:05

passed #143537
x86_64-linux
validate-x86_64-linux-fedora27

00:48:48

failed #143539
x86_64-windows allowed to fail
validate-x86_64-windows

02:36:06

failed #143538
x86_64-windows allowed to fail
validate-x86_64-windows-hadrian

02:38:13

 
  Cleanup
passed #143540
x86_64-darwin
cleanup-darwin

00:00:31

 
  Packaging
passed #143541
x86_64-linux
doc-tarball

00:07:07

 
  Testing
manual #143542
x86_64-linux allowed to fail manual
hackage
passed #143543
x86_64-linux
perf-nofib

00:19:42

 
Name Stage Failure
failed
validate-aarch64-linux-deb9 Full Build
$ cp -Rf $HOME/.cabal cabal-cache
Uploading artifacts...
ghc-*.tar.xz: found 1 matching files
junit.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=143533 responseStatus=201 Created token=xCnuaPiE
Uploading artifacts...
junit.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=143533 responseStatus=201 Created token=xCnuaPiE
ERROR: Job failed: exit code 1
failed
validate-x86_64-windows-hadrian Full Build
junit.xml: found 1 matching files                  
WARNING: Failed to load system CertPool:crypto/x509: system root pool is not available on Windows
Uploading artifacts to coordinator... ok
id=143538 responseStatus=201 Created token=HVGiHZvk
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=6552 revision=1f513601 version=11.10.1
junit.xml: found 1 matching files

WARNING: Failed to load system CertPool:crypto/x509: system root pool is not available on Windows
Uploading artifacts to coordinator... ok
id=143538 responseStatus=201 Created token=HVGiHZvk
ERROR: Job failed: exit status 1
failed
validate-x86_64-windows Full Build
junit.xml: found 1 matching files                  
WARNING: Failed to load system CertPool:crypto/x509: system root pool is not available on Windows
Uploading artifacts to coordinator... ok
id=143539 responseStatus=201 Created token=iUFgGzQF
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=4968 revision=1f513601 version=11.10.1
junit.xml: found 1 matching files

WARNING: Failed to load system CertPool:crypto/x509: system root pool is not available on Windows
Uploading artifacts to coordinator... ok
id=143539 responseStatus=201 Created token=iUFgGzQF
ERROR: Job failed: exit status 2