Fix broken PyImath.pc, and add pkg-config CI tests #1117

Job	Run time
Linux.3: static / steps	13m 52s
Linux.4: pkgconfig=OFF, testing=OFF, python=OFF / steps	1m 10s
Linux.6: clang / steps	13m 52s
Linux.5: custom namespace / steps	13m 52s
Linux.1: Release / steps	13m 53s
Linux.2: Debug / steps	13m 36s
macOS.3: static / steps	8m 26s
macOS.5: macos-13 / steps	8m 32s
macOS.6: Apple Frameworks / steps	31s
macOS.2: Debug / steps	8m 7s
macOS.1: Release / steps	8m 27s
Windows.1: Release / steps	21m 56s
macOS.4: pkgconfig=OFF, testing=OFF / steps	35s
Windows.3: static / steps	18m 40s
Windows.2: Debug / steps	4m 15s
Windows.6: msys2-mingw32 / steps	3m 41s
Windows.5: vfx2023 / steps	20m 52s
Windows.4: pkgconfig=OFF, testing=OFF / steps	1m 31s
Windows.8: msys2-ucrt64 / steps	4m 36s
Windows.9: msys2-ucrt64, static / steps	4m 29s
Windows.7: msys2-mingw32, static / steps	3m 35s
	3h 8m 28s

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