% gcc -v gg.c
Apple LLVM version 10.0.0 (clang-1000.11.45.2)
Target: x86_64-apple-darwin18.0.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
 "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang" -cc1 -triple x86_64-apple-macosx10.14.0 -Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name gg.c -mrelocation-model pic -pic-level 2 -mthread-model posix -mdisable-fp-elim -fno-strict-return -masm-verbose -munwind-tables -target-cpu penryn -dwarf-column-info -debugger-tuning=lldb -target-linker-version 409.12 -v -resource-dir /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/10.0.0 -fdebug-compilation-dir /Users/bwarsaw/projects/python -ferror-limit 19 -fmessage-length 131 -stack-protector 1 -fblocks -fencode-extended-block-signature -fobjc-runtime=macosx-10.14.0 -fmax-type-align=16 -fdiagnostics-show-option -o /var/folders/06/yd7czfp11bx1vx4p5dl10v4w000slb/T/gg-a38469.o -x c gg.c
clang -cc1 version 10.0.0 (clang-1000.11.45.2) default target x86_64-apple-darwin18.0.0
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/10.0.0/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include
 /usr/include
 /System/Library/Frameworks (framework directory)
 /Library/Frameworks (framework directory)
End of search list.
 "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld" -demangle -lto_library /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/libLTO.dylib -no_deduplicate -dynamic -arch x86_64 -macosx_version_min 10.14.0 -o a.out /var/folders/06/yd7czfp11bx1vx4p5dl10v4w000slb/T/gg-a38469.o -lSystem /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/10.0.0/lib/darwin/libclang_rt.osx.a

% otool -L a.out 
a.out:
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1252.200.5)

So I guess it's interesting that it's searching /usr/local/include, which has some Homebrew headers in it:

% ls /usr/local/include
ImageMagick-7@		gdcache.h@		libltdl@		pg_config_manual.h@	sqlda-native.h@
adns.h@			gdfontg.h@		libpng16@		pg_config_os.h@		sqlda.h@
aspell.h@		gdfontl.h@		libpq@			pgtypes.h@		tiff.h@
assuan.h@		gdfontmb.h@		libpq-events.h@		pgtypes_date.h@		tiffconf.h@
ecpg_config.h@		gdfonts.h@		libpq-fe.h@		pgtypes_error.h@	tiffio.h@
ecpg_informix.h@	gdfontt.h@		libssh2.h@		pgtypes_interval.h@	tiffio.hxx@
ecpgerrno.h@		gdfx.h@			libssh2_publickey.h@	pgtypes_numeric.h@	tiffvers.h@
ecpglib.h@		gdpp.h@			libssh2_sftp.h@		pgtypes_timestamp.h@	unicase.h@
ecpgtype.h@		gmp.h@			libtasn1.h@		png.h@			uniconv.h@
entities.h@		gmpxx.h@		libusb-1.0@		pngconf.h@		unictype.h@
evdns.h@		gnutls@			ltdl.h@			pnglibconf.h@		unigbrk.h@
event.h@		gpg-error.h@		lzma@			popt.h@			unilbrk.h@
event2@			gpgrt.h@		lzma.h@			postgres_ext.h@		uniname.h@
evhttp.h@		graphviz@		mysql@			pspell@			uninorm.h@
evrpc.h@		idn2.h@			nettle@			python3.6m/		unistdio.h@
evutil.h@		informix@		newt.h@			python3.7m/		unistr.h@
fontconfig@		internal@		npth.h@			python3.8m/		unistring@
freetype2@		jconfig.h@		nspr@			readtags.h@		unitypes.h@
gcrypt.h@		jerror.h@		openjpeg-2.3@		server@			uniwbrk.h@
gd.h@			jmorecfg.h@		p11-kit-1@		slang.h@		uniwidth.h@
gd_color_map.h@		jpeglib.h@		pcre2.h@		slcurses.h@		webp@
gd_errors.h@		ksba.h@			pcre2posix.h@		sql3types.h@
gd_io.h@		lcms2.h@		pg_config.h@		sqlca.h@
gdbm.h@			lcms2_plugin.h@		pg_config_ext.h@	sqlda-compat.h@

All those symlinks point into ../Cellar/

It doesn't look much different than what happens on 10.13.  I'll try to see what the paths are when I build Python.