(maint) Disable precompiled headers with MinGW

MinGW crashes on sufficiently large precompiled headers. We ran into this
after enabling them with Cotire. Disable precompiled headers when
compiling with MinGW.
Merge pull request #170 from MikaelSmith/maint/master/MinGW-disable-precompiled

(maint) Disable precompiled headers with MinGW