Release v0.7.0

Improve platform detection and simplify cross compilation

- detect target platform rather than build platform
- introduce optional user variable 'PLATFORM' for cross compilation
- no longer build OSX/MacOS fat binaries by default
- do build fat binaries when 'extension=d_fat' is specified for OSX/MacOS
- fix bug where minimum OSX/MacOS version wasn't defined, and set it to 10.6

Fixes and improvements for Windows builds

- properly evaluate variables 'PDDIR' and 'PDBINDIR' to find pd.dll
- define default path of 32 bit Pd on 64 bit Windows
- link C++ externals with standard C libs on Windows, they don't load otherwise
- strip installed Windows binaries by default

Warning for all platforms: variable 'PD_PATH' is no longer supported, use the
equivalent 'PDDIR'.

Implement target architecture detection for Windows 32/64 bit.

Use variable 'system' when evaluating 'for{Linux,Darwin,Windows}'

Replace flags '-fpic' by 'fPIC'

Fixed bug where message about m_pd.h is incorrect

Respect cflag for minimum OSX version from lib makefile

variables PDDIR PDINCLUDEDIR PDBINDIR PDLIBDIR

Version 0.4.0 introduces path variables with coherent names, and as
a bonus these paths can be specified in the environment. Legacy names
(PD_PATH, pdincludepath, pdbinpath, objectsdir) are deprecated but
still supported.

Fix bug where pd.dll wouldn't be found