If in Projucer under juce_audio_processors "JUCE_PLUGINHOST_VST" is checked, then you get JUCE/modules/juce_audio_processors/format_types/juce_VSTPluginFormat.cpp:51:10: fatal error: 'pluginterfaces/vst2.x/aeffect.h' file not found.
In this case it seems like you need to open this project in Projuicer, get https://archive.org/download/VST2SDK/vst_sdk2_4_rev2.zip, unzip it, and put the path to it in Projuicer under File -> Global Paths... -> VST (Legacy) SDK.
On FreeBSD:
If in Projucer under juce_audio_processors "JUCE_PLUGINHOST_VST" is checked, then you get
JUCE/modules/juce_audio_processors/format_types/juce_VSTPluginFormat.cpp:51:10: fatal error: 'pluginterfaces/vst2.x/aeffect.h' file not found. In this case it seems like you need to open this project in Projuicer, get https://archive.org/download/VST2SDK/vst_sdk2_4_rev2.zip, unzip it, and put the path to it in Projuicer under File -> Global Paths... -> VST (Legacy) SDK.