Makefile: parse TENDERMINT_BUILD_OPTIONS (#4738)

Reduce the number of targets and make the buildsystem more
flexible by parsing the TENDERMINT_BUILD_OPTIONS command
line variable (a-la Debian, inspired by dpkg-buildpackage's
DEB_BUILD_OPTIONS), e.g:

 $ make install TENDERMINT_BUILD_OPTIONS='cleveldb'

replaces the old:

 $ make install_c

Options can be mix&match'd, e.g.:

 $ make install TENDERMINT_BUILD_OPTIONS='cleveldb race nostrip'

Three options are available:
- nostrip: don't strip debugging symbols nor DWARF tables.
- cleveldb: use cleveldb as db backend instead of goleveldb;
  it switches on the CGO_ENABLED Go environment variale.
- race: pass -race to go build and enable data race detection.

This changeset is a port of gaia pull request: cosmos/gaia#363.

docs/introduction/install.md

@@ -119,13 +119,13 @@ db_backend = "cleveldb"
 To install Tendermint, run:
 
 ```
-CGO_LDFLAGS="-lsnappy" make install_c
+CGO_LDFLAGS="-lsnappy" make install TENDERMINT_BUILD_OPTIONS=cleveldb
 ```
 
 or run:
 
 ```
-CGO_LDFLAGS="-lsnappy" make build_c
+CGO_LDFLAGS="-lsnappy" make build TENDERMINT_BUILD_OPTIONS=cleveldb
 ```
 
 which puts the binary in `./build`.