Fix include flags order

AM_CPPFLAGS is the correct variable to use for preprocessor flags. Importantly, it is placed on the compiler command line before CPPFLAGS, whereas AM_CFLAGS comes after CPPFLAGS. So before this change, it was possible for incorrect headers in external locations to be used instead of the internal ones specified in SET_INCLUDES.
4 jobs for patch-1 in 1 minute and 30 seconds
Status Job ID Name Coverage
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passed #415578
amd64 debian
build-debian

00:00:34

passed #415579
macos
build-macos

00:01:17

passed #415581
amd64 docker
build-win32

00:01:24

passed #415580
amd64 docker
build-win64

00:01:30