Using openmp on windows with mingw. Cannot find -lpthread

I have a CMake project which is using OpenMP and works on linux. When I went to compile it on my windows machine it looked like CMake was having trouble finding the openmp flags for mingw's gcc.

I decided to try a smaller test case and just compile main_openmp.c

#include <omp.h>#include <stdio.h>int main(int argc, char* argv[]) {    int id;    #pragma omp parallel private(id)        {        id = omp_get_thread_num();        printf("%d: Hello World!\n", id);        }    return 0;}

Then when I try to compile

gcc -o OpenMPTest2 main_testomp.c -fopenmp

I get

>>> gcc -o OpenMPTest2 main_testomp.c -fopenmpc:/mingw/bin/../lib/gcc/mingw32/4.8.1/../../../../mingw32/bin/ld.exe: cannot find -lpthreadcollect2.exe: error: ld returned 1 exit status

I attempted to find the solution online and tried variants of -lgomp, -lpthreadgc2, and -lpthreadvc2, with no improvement.

I searched my C:\MinGw directory recursively for any filenames containing lpthread and got this:

C:\MinGW\bin\pthreadgc2.dllC:\MinGW\bin\pthreadgce2.dllC:\MinGW\var\cache\mingw-get\packages\pthreads-w32-2.9.1-1-mingw32-dll.tar.lzmaC:\MinGW\var\lib\mingw-get\data\mingw32-pthreads-w32.xml

I'm not sure if I'm missing a flag, or a package, or what I'm doing wrong. For good measure here is the output of gcc -v

Using built-in specs.COLLECT_GCC=gccCOLLECT_LTO_WRAPPER=c:/mingw/bin/../libexec/gcc/mingw32/4.8.1/lto-wrapper.exeTarget: mingw32Configured with: ../gcc-4.8.1/configure --prefix=/mingw --host=mingw32 --build=mingw32 --without-pic --enable-shared --enable-static --with-gnu-ld --enable-lto --enable-libssp --disable-multilib --enable-languages=c,c++,fortran,objc,obj-c++,ada --disable-sjlj-exceptions --with-dwarf2 --disable-win32-registry --enable-libstdcxx-debug --enable-version-specific-runtime-libs --with-gmp=/usr/src/pkg/gmp-5.1.2-1-mingw32-src/bld --with-mpc=/usr/src/pkg/mpc-1.0.1-1-mingw32-src/bld --with-mpfr= --with-system-zlib --with-gnu-as --enable-decimal-float=yes --enable-libgomp --enable-threads --with-libiconv-prefix=/mingw32 --with-libintl-prefix=/mingw --disable-bootstrap LDFLAGS=-s CFLAGS=-D_USE_32BIT_TIME_TThread model: win32gcc version 4.8.1 (GCC)

Any idea what's wrong?