On FBSD 12-STABLE (FreeBSD 12.0-STABLE #257 r342843: Mon Jan  7 08:01:54 CET 2019 amd64) running PostgreSQL-11 server (most recent /usr/ports tree, psql server recompiled!), updating postgis22 fails with this compilation error:

[...]
gmake[4]: Entering directory '/usr/ports/databases/postgis22/work/postgis-2.2.6/libpgcommon'
cc -I../liblwgeom -O2 -pipe  -I/usr/local/include -fstack-protector -fno-strict-aliasing  -I/usr/local/include/postgresql/server   -fPIC -DPIC  -Wall -Wmissing-prototypes  -c -o gserialized_gist.o gserialized_gist.c
In file included from gserialized_gist.c:22:
./lwgeom_pg.h:135:5: error: token is not a valid binary operator in a preprocessor subexpression
#if POSTGIS_PGSQL_VERSION >= 85
    ^~~~~~~~~~~~~~~~~~~~~
./../postgis_config.h:150:42: note: expanded from macro 'POSTGIS_PGSQL_VERSION'
#define POSTGIS_PGSQL_VERSION PostgreSQL 11PostgreSQL 11
                              ~~~~~~~~~~ ^
gserialized_gist.c:98:25: error: use of undeclared identifier 'FLT_MAX'
                                GIDX_SET_MIN(a,2,-1*FLT_MAX);
                                                    ^
gserialized_gist.c:99:22: error: use of undeclared identifier 'FLT_MAX'
                                GIDX_SET_MAX(a,2,FLT_MAX);
                                                 ^
gserialized_gist.c:277:27: error: use of undeclared identifier 'FLT_MAX'
                        GIDX_SET_MIN(gidx,2,-1*FLT_MAX);
                                               ^
gserialized_gist.c:278:24: error: use of undeclared identifier 'FLT_MAX'
                        GIDX_SET_MAX(gidx,2,FLT_MAX);
                                            ^
5 errors generated.