CI: Add a script for building the latest GMT source codes (#3193)

Other projects can use the following command to install GMT:

```
curl https://raw.githubusercontent.com/GenericMappingTools/gmt/master/ci/build-gmt-master.sh | bash
```
See GenericMappingTools/custom-supplements#3 for an example TravisCI configuration file.

Author: seisman

ci/build-gmt-master.sh

@@ -0,0 +1,62 @@
+#!/usr/bin/env bash
+#
+# Build the lastest GMT source codes.
+#
+# Usage:
+#
+# 1. Install GMT dependencies following the [wiki](https://github.com/GenericMappingTools/gmt/wiki)
+# 2. curl https://raw.githubusercontent.com/GenericMappingTools/gmt/master/ci/build-gmt-master.sh | bash
+#
+# Environmental variables that affect the building process:
+#
+# - GMT_INSTALL_DIR: GMT installation location
+#
+
+set -x -e
+
+# Following variables can be modified via environment variables
+GMT_INSTALL_DIR=${GMT_INSTALL_DIR:-${HOME}/gmt-install-dir}
+
+# General settings
+GSHHG_VERSION="2.3.7"
+DCW_VERSION="1.1.4"
+GSHHG="gshhg-gmt-${GSHHG_VERSION}"
+DCW="dcw-gmt-${DCW_VERSION}"
+EXT="tar.gz"
+
+cwd=${PWD}
+# 1. Create temporary directory for building
+GMT_BUILD_TMPDIR=$(mktemp -d ${TMPDIR:-/tmp/}gmt.XXXXXX)
+cd ${GMT_BUILD_TMPDIR}
+
+# 2. Download GMT, GSHHG and DCW from GitHub
+git clone --depth 1 https://github.com/GenericMappingTools/gmt.git gmt
+curl -SLO https://github.com/GenericMappingTools/gshhg-gmt/releases/download/${GSHHG_VERSION}/${GSHHG}.${EXT}
+curl -SLO https://github.com/GenericMappingTools/dcw-gmt/releases/download/${DCW_VERSION}/${DCW}.${EXT}
+
+# 3. Extract tarballs
+tar -xvf ${GSHHG}.${EXT}
+tar -xvf ${DCW}.${EXT}
+mv ${GSHHG} gmt/share/gshhg-gmt
+mv ${DCW} gmt/share/dcw-gmt
+
+# 4. Configure GMT
+cd gmt/
+cat > cmake/ConfigUser.cmake << EOF
+set (CMAKE_INSTALL_PREFIX "${GMT_INSTALL_DIR}")
+set (CMAKE_C_FLAGS "-Wall -Wdeclaration-after-statement \${CMAKE_C_FLAGS}")
+set (CMAKE_C_FLAGS "-Wextra \${CMAKE_C_FLAGS}")
+EOF
+
+# 5. Build and install GMT
+mkdir build
+cd build
+cmake ..
+make
+make install
+
+# 6. Cleanup
+cd ${cwd}
+rm -rf ${GMT_BUILD_TMPDIR}
+
+set -x -e