[NFC][cmake] Build fix in tools/llvm-config/CMakeLists.txt

To avoid the following error message (using cmake version 3.13.4) :

```
CMake Error at tools/llvm-config/CMakeLists.txt:37 (string):
Syntax error in cmake code  when parsing string-std=[^ ]\+Invalid escape sequence \+
```

Reviewed By: mgorny

Differential Revision: https://reviews.llvm.org/D58619



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369887 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-config/CMakeLists.txt

@@ -34,9 +34,9 @@ get_property(COMPILE_FLAGS TARGET llvm-config PROPERTY COMPILE_FLAGS)
 # NOTE: We don't want to start extracting any random C/CXX flags that the
 # user may add that could affect the ABI.  We only want to extract flags
 # that have been added by the LLVM build system.
-string(REGEX MATCH "-std=[^ ]\+" LLVM_CXX_STD_FLAG ${CMAKE_CXX_FLAGS})
-string(REGEX MATCH "-stdlib=[^ ]\+" LLVM_CXX_STDLIB_FLAG ${CMAKE_CXX_FLAGS})
-string(REGEX MATCH "-std=[^ ]\+" LLVM_C_STD_FLAG ${CMAKE_C_FLAGS})
+string(REGEX MATCH "-std=[^ ]+" LLVM_CXX_STD_FLAG ${CMAKE_CXX_FLAGS})
+string(REGEX MATCH "-stdlib=[^ ]+" LLVM_CXX_STDLIB_FLAG ${CMAKE_CXX_FLAGS})
+string(REGEX MATCH "-std=[^ ]+" LLVM_C_STD_FLAG ${CMAKE_C_FLAGS})
 
 # Use configure_file to create BuildVariables.inc.
 set(LLVM_SRC_ROOT ${LLVM_MAIN_SRC_DIR})