Fix link error without static library

[fd00]

$ git clone https://github.com/microsoft/mimalloc.git
$ cd mimalloc
$ mkdir build
$ cmake -DMI_BUILD_STATIC:BOOL=OFF ..
$ make
:
[ 78%] Linking C executable mimalloc-test-api
ld: library 'mimalloc-static' not found
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [mimalloc-test-api] Error 1
make[1]: *** [CMakeFiles/mimalloc-test-api.dir/all] Error 2
make: *** [all] Error 2