[OS:Win10] Getting "Failed to determine the source files for the regular expression backend" on fresh copy from git when running cmake

Following the steps in the readme file (text below), when doing cmake ../ I get: "Failed to determine the source files for the regular expression backend". I did this a month ago without issue. This was done on a Windows 10 PC with the latest versions of cmake and VS as of early June.

```
# Check out the library.
$ git clone https://github.com/google/benchmark.git
# Benchmark requires Google Test as a dependency. Add the source tree as a subdirectory.
$ git clone https://github.com/google/googletest.git benchmark/googletest\
# Go to the library root directory
$ cd benchmark
# Make a build directory to place the build output.
$ mkdir build && cd build
# Generate a Makefile with cmake.
# Use cmake -G to generate a different file type.
$ cmake ../
# Build the library.
# Use make -j to speed up the build process, e.g. make -j8 .
$ make
```