Linux: no Cairo renderer since #7889 (ofBeginSaveScreenAsPDF / SVG missing, pdfExample doesn't compile) #8608

Since #7889, ofCairoRenderer.h / .cpp and ofGraphicsCairo.h / .cpp are wrapped in #if defined(OF_CAIRO), but ofConstants.h defines OF_CAIRO only for TARGET_WIN32 and macOS, not TARGET_LINUX. So on Linux the Cairo renderer and ofBeginSaveScreenAsPDF / ofEndSaveScreenAsPDF / ofBeginSaveScreenAsSVG / ofEndSaveScreenAsSVG compile to nothing, even though the Linux makefiles still link Cairo (PLATFORM_PKG_CONFIG_LIBRARIES += cairo). This is in 0.12.1 too.

examples/input_output/pdfExample fails on Linux:

src/ofApp.cpp:33:17: error: 'ofBeginSaveScreenAsPDF' was not declared in this scope
src/ofApp.cpp:125:17: error: 'ofEndSaveScreenAsPDF' was not declared in this scope

Seen on Ubuntu 24.04 on master a7ede0a5d, in an all-examples build (#8605). Defining OF_CAIRO for TARGET_LINUX brings it back.