[build] Downgrade GCC support from 5.0 to 4.9

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+# Modern C++ support
+
+Mapbox GL Native supports the C++14 standard, and encourages contributions to
+the source code using modern C++ idioms like return type deductions, generic
+lambdas, `std::optional` and alike. However, we do not support all the features
+from the final draft of the C++14 standard - we had to sacrifice support for
+these features in order to support GCC from version 4.9 onwards.
+
+The following C++14 features are **not supported** in Mapbox GL Native:
+
+## [C++14 variable templates](https://isocpp.org/wiki/faq/cpp14-language#variable-templates)
+
+Constructs like the example below are not supported:
+
+```C++
+template<typename T> constexpr T pi = T(3.14);
+```
+
+### Workarounds:
+
+- If the variable is an alias, use the call the alias points to: [example](https://github.com/mapbox/mapbox-gl-native/commit/f1ac757bd28351fd57113a1e16f6c2e00ab193c1#diff-711ce10b54a522c948efc9030ffab4fcL269)
+```C++
+// auto foo = pi<double>;
+auto foo = double(3.14);
+```
+
+- Replace variable templates with either functions or structs: [example 1](https://github.com/mapbox/mapbox-gl-native/commit/f1ac757bd28351fd57113a1e16f6c2e00ab193c1#diff-ffbe6cdfd30513aaa4749b4d959a5da6L58), [example 2](https://github.com/mapbox/mapbox-gl-native/commit/f1ac757bd28351fd57113a1e16f6c2e00ab193c1#diff-04af54dc8685cdc382ebe24466dc1d00L98)
+
+## [C++14 aggregates with non-static data member initializers](http://en.cppreference.com/w/cpp/language/aggregate_initialization)
+
+Constructs like the example below are not supported:
+
+```C++
+struct Foo {
+    int x = { 0 };
+};
+
+// error: no matching function for call to 'Foo::Foo(<brace-enclosed initializer list>)'
+int main() {
+    Foo foo { 0 };
+    return 0;
+}
+```
+
+### Workarounds
+- Replace data member initializers with default parameter values in default constructors:
+
+```C++
+struct Foo {
+    Foo(int x_ = 0) : x(x_) {}
+    int x;
+};
+
+int main() {
+    Foo foo { 0 }; // works with default constructor
+    return 0;
+}
+```
+
+- Replace bracket initialization with regular round brackets or none:
+
+```C++
+struct Foo {
+    Foo(int x_ = 0) : x(x_) {}
+    int x;
+};
+
+int main() {
+    Foo foo(); // works
+    Foo bar; // also works
+    return 0;
+}
+```
+
+## [Extended `constexpr` support](https://isocpp.org/wiki/faq/cpp14-language#extended-constexpr)
+
+GCC 4.9 strictly forbids `constexpr` usage in the following scenarios:
+- No local variable declarations (not `static` or `thread_local`, and no uninitialized variables)
+- Cannot mutate objects whose lifetime began with the constant expression evaluation
+- Disable usage of if, switch, for, while, do-while (not goto) inside constexpr expressions
+- Enforces that constexpr member functions are implicitly const
+
+```C++
+// sorry, unimplemented: use of the value of the object being constructed
+// in a constant expression
+struct Foo {
+    int x, y;
+    constexpr Foo(int i) : x(i), y(x) {}
+};
+
+// error: body of constexpr function 'constexpr int test1(int)' not a
+// return-statement
+constexpr int test1(int i) {
+    int j = i;
+    return j;
+}
+
+// error: body of constexpr function 'constexpr bool test2(int)' not a
+// return-statement
+constexpr bool test2(int i) {
+    if (i > 0) {
+        return true;
+    } else {
+        return false;
+    }
+}
+```
+
+### Workarounds
+
+- Either remove `constexpr` specifier or replace it with `inline` in case of
+  functions