build: improve regsub pattern for setting buildToolsVersion

This addresses the discussion in !64
https://gitlab.com/fdroid/fdroidserver/merge_requests/64

Sometimes, buildToolsVersion is a kind of gradle macro call, and other
times it is a variable assignment.  This regsub pattern now handles both of
those cases.

tests/source-files/Zillode/syncthing-silk/build.gradle

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2015 OpenSilk Productions LLC
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+// Top-level build file where you can add configuration options common to all sub-projects/modules.
+
+buildscript {
+    repositories {
+        mavenCentral()
+        jcenter()
+    }
+    dependencies {
+        classpath 'com.android.tools.build:gradle:1.1.3'
+        classpath 'me.tatarka:gradle-retrolambda:2.5.0'
+        classpath 'org.robolectric:robolectric-gradle-plugin:1.0.1'
+
+        // NOTE: Do not place your application dependencies here; they belong
+        // in the individual module build.gradle files
+    }
+}
+
+allprojects {
+    repositories {
+        mavenCentral()
+        jcenter()
+        maven { url '../../m2/repository' }
+        maven { url 'https://oss.sonatype.org/content/repositories/snapshots' }
+    }
+}
+
+// Build config
+ext.compileSdkVersion = 22
+ext.buildToolsVersion  = "22.0.1"
+
+// defaultConfig
+ext.targetSdkVersion = 22
+
+ext.supportLibVersion = "22.1.1"
+ext.dagger2Version = "2.0"
+ext.rxAndroidVersion = "0.23.0"
+ext.timberVersion = "2.5.0"
+ext.commonsLangVersion = "3.3.2"
+ext.butterKnifeVersion = "6.0.0"
+ext.commonsIoVersion = "2.4"
+ext.gsonVersion = "2.3"
+
+def gitSha() {
+    return 'git rev-parse --short HEAD'.execute().text.trim()
+}
+
+def getDebugVersionSuffix() {
+    return "${gitSha()}".isEmpty() ? "-SNAPSHOT" : "-SNAPSHOT-${gitSha()}"
+}