diff --git a/configure b/configure
index 7029e036269..cf2d83b93ad 100755
--- a/configure
+++ b/configure
@@ -7593,7 +7593,7 @@ fi
 # On Solaris, we need these #defines to get POSIX-conforming versions
 # of many interfaces (sigwait, getpwuid_r, shmdt, ...).
 if test "$PORTNAME" = "solaris"; then
-  CPPFLAGS="$CPPFLAGS -D_POSIX_C_SOURCE=200112L -D_POSIX_PTHREAD_SEMANTICS"
+  CPPFLAGS="$CPPFLAGS -D_POSIX_C_SOURCE=200112L -D__EXTENSIONS__ -D_POSIX_PTHREAD_SEMANTICS"
 fi
 
 # We already have this in Makefile.win32, but configure needs it too
diff --git a/configure.ac b/configure.ac
index afc229e729b..399aae4eed9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -696,7 +696,7 @@ fi
 # On Solaris, we need these #defines to get POSIX-conforming versions
 # of many interfaces (sigwait, getpwuid_r, shmdt, ...).
 if test "$PORTNAME" = "solaris"; then
-  CPPFLAGS="$CPPFLAGS -D_POSIX_C_SOURCE=200112L -D_POSIX_PTHREAD_SEMANTICS"
+  CPPFLAGS="$CPPFLAGS -D_POSIX_C_SOURCE=200112L -D__EXTENSIONS__ -D_POSIX_PTHREAD_SEMANTICS"
 fi
 
 # We already have this in Makefile.win32, but configure needs it too
diff --git a/meson.build b/meson.build
index ec5cd9259df..7d9a247d21c 100644
--- a/meson.build
+++ b/meson.build
@@ -263,7 +263,11 @@ elif host_system == 'sunos'
   export_fmt = '-Wl,-M@0@'
   # We need these #defines to get POSIX-conforming versions
   # of many interfaces (sigwait, getpwuid_r, shmdt, ...).
-  cppflags += [ '-D_POSIX_C_SOURCE=200112L', '-D_POSIX_PTHREAD_SEMANTICS' ]
+  cppflags += [
+    '-D_POSIX_C_SOURCE=200112L',
+    '-D__EXTENSIONS__',
+    '-D_POSIX_PTHREAD_SEMANTICS',
+  ]
 
 elif host_system == 'windows'
   portname = 'win32'