Add compiler.has_define

Adds a new method to the compiler object, has_define.
This makes it possible to check if a preprocessor macro/define
is set or not.

This is especially helpful if the define in question is empty,
for example:

  #define MESON_EMPTY_DEFINE

This would yield the same results as a missing define with
the existing get_define method, as it would return an empty
string for both cases. Therefore this additional method is
needed.

1 files changed, 10 insertions, 0 deletions

diff --git a/docs/markdown/snippets/compiler_has_define.md b/docs/markdown/snippets/compiler_has_define.md
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index 0000000..64de26b
--- /dev/null
+++ b/docs/markdown/snippets/compiler_has_define.md
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+## Compilers now have a `has_define` method
+
+This method returns true if the given preprocessor symbol is
+defined, else false is returned. This is useful is cases where
+an empty define has to be distinguished from a non-set one, which
+is not possible using `get_define`.
+
+Additionally it makes intent clearer for code that only needs
+to check if a specific define is set at all and does not care
+about its value.
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