doc: mention MAKE=gmake workaround when building on a BSD

Fixes: #14404

doc/build-netbsd.md

@@ -37,13 +37,13 @@ from ports, for the same reason as boost above (g++/libstd++ incompatibility).
 If you have to build it yourself, you can use [the installation script included
 in contrib/](/contrib/install_db4.sh) like so:
 
-```shell
+```bash
 ./contrib/install_db4.sh `pwd`
 ```
 
 from the root of the repository. Then set `BDB_PREFIX` for the next section:
 
-```shell
+```bash
 export BDB_PREFIX="$PWD/db4"
 ```
 
@@ -52,24 +52,26 @@ export BDB_PREFIX="$PWD/db4"
 **Important**: Use `gmake` (the non-GNU `make` will exit with an error).
 
 With wallet:
-```
+```bash
 ./autogen.sh
 ./configure --with-gui=no CPPFLAGS="-I/usr/pkg/include" \
     LDFLAGS="-L/usr/pkg/lib" \
     BOOST_CPPFLAGS="-I/usr/pkg/include" \
     BOOST_LDFLAGS="-L/usr/pkg/lib" \
     BDB_LIBS="-L${BDB_PREFIX}/lib -ldb_cxx-4.8" \
-    BDB_CFLAGS="-I${BDB_PREFIX}/include"
+    BDB_CFLAGS="-I${BDB_PREFIX}/include" \
+    MAKE=gmake
 ```
 
 Without wallet:
-```
+```bash
 ./autogen.sh
 ./configure --with-gui=no --disable-wallet \
     CPPFLAGS="-I/usr/pkg/include" \
     LDFLAGS="-L/usr/pkg/lib" \
     BOOST_CPPFLAGS="-I/usr/pkg/include" \
-    BOOST_LDFLAGS="-L/usr/pkg/lib"
+    BOOST_LDFLAGS="-L/usr/pkg/lib" \
+    MAKE=gmake
 ```
 
 Build and run the tests: