Incorrect usage of _MSC_VER

[meshula]

User @Biswa96 has reported that we are using _MSC_VER (detect the usage of the Microsoft Visual C compiler) where we are intending to use _WIN32 (detect that the target platform is Windows). We should check the uses of _MSC_VER and correct them as necessary. Thank you @bisaw96 for the report.

[kmilos]

OTOH, you'd also want to enable IMATH_INSTALL_PKG_CONFIG for MINGW, but not for MSVC builds:

Imath/config/ImathSetup.cmake

Lines 39 to 46 in 3206446

	# Whether to generate and install a pkg-config file Imath.pc on
	if(WIN32)
	option(IMATH_INSTALL_PKG_CONFIG "Install Imath.pc file" OFF)
	option(IMATH_INSTALL_SYM_LINK "Create symbolic links for shared objects" OFF)
	else()
	option(IMATH_INSTALL_PKG_CONFIG "Install Imath.pc file" ON)
	option(IMATH_INSTALL_SYM_LINK "Create symbolic links for shared objects" ON)
	endif()

[Biswa96]

pkgconfig can be enabled with IMATH_INSTALL_PKG_CONFIG option. So, I didn't think to change that default behavior.

[kmilos]

Yeah, I meant "by default", not a big deal indeed.

[cary-ilm]

Should be resolved by #245.