编译PG插件时，报"unrecognized command line option ‘-Wimplicit-fallthrough=3’"。
关于implicit-fallthrough选项：
As of GCC 7, -Wimplicit-fallthrough is supported and __attribute__((fallthrough)) can be used to suppress the warnings when fallthrough is intentional. GCC does recognize "fallthrough" comments in certain scenarios, but it can be confused fairly easily.
而编译用的是gcc (GCC) 4.8.5，且没有地方指定了该选项。
唯一原因就是pg本身使用gcc 7编译的，可通过pg_config来确定编译用的选项。
[zjh@lightdb1 zhparser-0.2.0]$ pg_config BINDIR = /home/zjh/pgsql13.2/bin DOCDIR = /home/zjh/pgsql13.2/share/doc HTMLDIR = /home/zjh/pgsql13.2/share/doc INCLUDEDIR = /home/zjh/pgsql13.2/include PKGINCLUDEDIR = /home/zjh/pgsql13.2/include INCLUDEDIR-SERVER = /home/zjh/pgsql13.2/include/server LIBDIR = /home/zjh/pgsql13.2/lib PKGLIBDIR = /home/zjh/pgsql13.2/lib LOCALEDIR = /home/zjh/pgsql13.2/share/locale MANDIR = /home/zjh/pgsql13.2/share/man SHAREDIR = /home/zjh/pgsql13.2/share SYSCONFDIR = /home/zjh/pgsql13.2/etc PGXS = /home/zjh/pgsql13.2/lib/pgxs/src/makefiles/pgxs.mk CONFIGURE = '--prefix=/home/zjh/pgsql13.2' '--enable-depend' '--enable-cassert' '--enable-debug' 'CFLAGS=-DLOCK_DEBUG -DBTREE_BUILD_STATS -DWAL_DEBUG -DCATCACHE_STATS -ggdb -O0' '--enable-dtrace' '--enable-nls=gbk' '--with-openssl' '--with-libxml' '--with-zlib' '--with-llvm' 'LLVM_CONFIG=/opt/rh/llvm-toolset-7/root/usr/bin/llvm-config' '--enable-profiling' CC = gcc CPPFLAGS = -D_GNU_SOURCE -I/usr/include/libxml2 CFLAGS = -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wimplicit-fallthrough=3 -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -Wno-format-truncation -g -pg -DLINUX_PROFILE -DLOCK_DEBUG -DBTREE_BUILD_STATS -DWAL_DEBUG -DCATCACHE_STATS -ggdb -O0 CFLAGS_SL = -fPIC LDFLAGS = -L/opt/rh/llvm-toolset-7/root/usr/lib64 -Wl,--as-needed -Wl,-rpath,'/home/zjh/pgsql13.2/lib',--enable-new-dtags LDFLAGS_EX = LDFLAGS_SL = LIBS = -lpgcommon -lpgport -lpthread -lxml2 -lssl -lcrypto -lz -lreadline -lrt -ldl -lm VERSION = PostgreSQL 13.2
遗憾的是，没有pg_config选项可以修改其配置。
所以，要么用gcc 4.8.5重新编译pg，要么切换到7.3.1和pg_config的配置一致，例如llvm，可以从别的环境拷贝一个过来。
4.8.5重新编译后的选项如下：
[zjh@lightdb1 postgresql-13.2]$ pg_config BINDIR = /home/zjh/pgsql13.2/bin DOCDIR = /home/zjh/pgsql13.2/share/doc HTMLDIR = /home/zjh/pgsql13.2/share/doc INCLUDEDIR = /home/zjh/pgsql13.2/include PKGINCLUDEDIR = /home/zjh/pgsql13.2/include INCLUDEDIR-SERVER = /home/zjh/pgsql13.2/include/server LIBDIR = /home/zjh/pgsql13.2/lib PKGLIBDIR = /home/zjh/pgsql13.2/lib LOCALEDIR = /home/zjh/pgsql13.2/share/locale MANDIR = /home/zjh/pgsql13.2/share/man SHAREDIR = /home/zjh/pgsql13.2/share SYSCONFDIR = /home/zjh/pgsql13.2/etc PGXS = /home/zjh/pgsql13.2/lib/pgxs/src/makefiles/pgxs.mk CONFIGURE = '--prefix=/home/zjh/pgsql13.2' CC = gcc -std=gnu99 CPPFLAGS = -D_GNU_SOURCE CFLAGS = -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -O2 CFLAGS_SL = -fPIC LDFLAGS = -Wl,--as-needed -Wl,-rpath,'/home/zjh/pgsql13.2/lib',--enable-new-dtags LDFLAGS_EX = LDFLAGS_SL = LIBS = -lpgcommon -lpgport -lpthread -lz -lreadline -lrt -ldl -lm VERSION = PostgreSQL 13.2
重新编译就不报错了。