Revert "as discussed with @artemp - there should be no need or benefit to trying to handle F type in dbf since it is a foxpro extension that shapefiles do not commonly use"

This reverts commit 53acae94be4f5888f4f3d338fcc8fab7cc3b2041.
fix handling of F and O type dbf fields and sync handling for attribute_descriptors - refs #1614 and closes #1976
MinGW support for SConstruct script. Build now properly detects MinGW and finds dependencies correctly.
format update
Properly Initialize mingwbuild variable.
Added singleton unit test to check for multiple instances across multiple dynamic libraries.
Updates for supporting 64bit MinGW build on Windows 7+ via GNU tools.
Final MinGW 64 bit enhancements for Windows 7+ and Server 2008+