testsuite: Detect clang
testsuite: Detect clang

Clang doesn't like the -Wno-psabi argument that we want to pass to GCC.
Since clang is detected as GCC via __GNUC__, use ax_cv_c_compiler_vendor.

biosbits/libffi,rth7680/libffi,rth7680/libffi,joshtriplett/libffi,plicease/libffi,bgarrels/libffi,Pan7/libffi,biosbits/libffi,joshtriplett/libffi,Distrotech/libffi,joshtriplett/libffi,Pan7/libffi,Distrotech/libffi,biosbits/libffi,nmav/libffi,s1341/libffi,Distrotech/libffi,plicease/libffi,s1341/libffi,s1341/libffi,plicease/libffi,nmav/libffi,plicease/libffi,bgarrels/libffi,s1341/libffi,Pan7/libffi,joshtriplett/libffi,rth7680/libffi,rth7680/libffi,bgarrels/libffi,bgarrels/libffi,Pan7/libffi,Distrotech/libffi,biosbits/libffi,nmav/libffi,nmav/libffi