Boost 1.67.0: MinGW GCC 7.2.0 b2 error

`b2` error while trying to configure Boost 1.67.0 with MinGW.

Steps to reproduce:
```
[boost_1_67_0]> bootstrap.bat gcc
```

```
[boost_1_67_0]> b2 target-os=windows -a link=static threading=multi variant=debug,release --layout=tagged toolset=gcc --user-config=C:/.../boost_1_67_0/boost.user.jam --with-system
```

Where `boost.user.jam` content:

```
using gcc
:
: "C:/.../mingw-w64/x86_64-7.2.0-posix-seh-rt_v5-rev1/mingw64/bin/g++.exe" -std=c++17
: "C:/.../mingw-w64/x86_64-7.2.0-posix-seh-rt_v5-rev1/mingw64/bin/ar.exe"
"C:/.../mingw-w64/x86_64-7.2.0-posix-seh-rt_v5-rev1/mingw64/bin/ranlib.exe"
;
```

Error message:
```
'C:/.../mingw-w64/x86_64-7.2.0-posix-seh-rt_v5-rev1/mingw64/bin/g++.exe" "-std' is not recognized as an internal or external command, operable program or batch file.
'C:/.../mingw-w64/x86_64-7.2.0-posix-seh-rt_v5-rev1/mingw64/bin/g++.exe" "-std' is not recognized as an internal or external command, operable program or batch file.
'C:/.../mingw-w64/x86_64-7.2.0-posix-seh-rt_v5-rev1/mingw64/bin/g++.exe" "-std' is not recognized as an internal or external command, operable program or batch file.
'C:/.../mingw-w64/x86_64-7.2.0-posix-seh-rt_v5-rev1/mingw64/bin/g++.exe" "-std' is not recognized as an internal or external command, operable program or batch file.
C:/.../boost_1_67_0/tools/build/src/tools\gcc.jam:279: in gcc.init
*** argument error
* rule regex.split ( string separator )
* called with: ( [.] )
* missing argument separator
(builtin):see definition of rule 'regex.split' being called
C:/.../boost_1_67_0/tools/build/src/build\toolset.jam:44: in toolset.using
C:/.../boost_1_67_0/tools/build/src/build\project.jam:1052: in using
C:\...\boost_1_67_0\boost.user.jam:6: in modules.load
C:/.../boost_1_67_0/tools/build/src\build-system.jam:255: in load-config
C:/.../boost_1_67_0/tools/build/src\build-system.jam:449: in load-configuration-files
C:/.../boost_1_67_0/tools/build/src\build-system.jam:607: in load
C:\...\boost_1_67_0\tools\build\src/kernel\modules.jam:295: in import
C:\...\boost_1_67_0\tools\build\src/kernel/bootstrap.jam:139: in boost-build
C:\...\boost_1_67_0\boost-build.jam:17: in module scope
```

Regression note: Same configuration worked fine with Boost 1.64.0