diff --git a/srcpkgs/grass/patches/c99-bool.patch b/srcpkgs/grass/patches/c99-bool.patch
new file mode 100644
index 00000000000..ce0cdfcd692
--- /dev/null
+++ b/srcpkgs/grass/patches/c99-bool.patch
@@ -0,0 +1,157 @@
+From 1e0c11f526f64755f877957ce6c14b2159c3823f Mon Sep 17 00:00:00 2001
+From: nilason <n_larsson@yahoo.com>
+Date: Wed, 19 May 2021 11:38:49 +0200
+Subject: [PATCH] libgis: Enable the C99 bool type (#1567)
+
+Enables globally the C99 'bool' type with its values 'true' and 'false'.
+
+The old boolean constants 'TRUE' and 'FALSE' are still valid but
+redefined to 'true' and 'false' to emphasize their equivalence.
+
+(Re-)definitions of TRUE/FALSE in modules are removed.
+
+This change was originally triggered by a conflict of v.hull -- defining
+a local bool type -- with GDAL 3.3 which introduced the inclusion of
+stdbool.h (#1563).
+---
+ lib/lidar/lidar.h            |  3 ---
+ raster/r.param.scale/param.h |  2 --
+ raster/r.surf.idw/main.h     |  2 --
+ raster3d/r3.showdspf/Ball.c  |  2 --
+ raster3d/r3.showdspf/togif.c |  3 ---
+ vector/v.hull/chull.c        | 20 ++++++++------------
+ 7 files changed, 14 insertions(+), 27 deletions(-)
+
+diff --git a/lib/lidar/lidar.h b/lib/lidar/lidar.h
+index 89a25de3bf..978894ea74 100644
+--- a/lib/lidar/lidar.h
++++ b/lib/lidar/lidar.h
+@@ -62,9 +62,6 @@
+     /* INTERPOLATOR */
+ #define P_BILINEAR 		1
+ #define P_BICUBIC 		0
+-    /* Boolean definitions */
+-#define TRUE 			1
+-#define FALSE 			0
+ 
+ /*----------------------------------------------------------------------------------------------------------*/
+     /*STRUCTS DECLARATION */
+diff --git a/raster/r.param.scale/param.h b/raster/r.param.scale/param.h
+index 9c1dab4e81..2512fedaa2 100644
+--- a/raster/r.param.scale/param.h
++++ b/raster/r.param.scale/param.h
+@@ -18,8 +18,6 @@
+ 				/* 'blank' edge around raster.          */
+ #define MAX_WSIZE 499		/* Maximum dimensions of window.        */
+ 				/* Some useful labels.                  */
+-#define TRUE 1
+-#define FALSE 0
+ 
+ #define RAD2DEG M_R2D
+ #define DEG2RAD M_D2R
+diff --git a/raster/r.surf.idw/main.h b/raster/r.surf.idw/main.h
+index 33164f43cb..47028e7444 100644
+--- a/raster/r.surf.idw/main.h
++++ b/raster/r.surf.idw/main.h
+@@ -1,8 +1,6 @@
+ #include <grass/raster.h>
+ 
+ #define         SHORT           short
+-#define         TRUE    1
+-#define         FALSE   0
+ 
+ #define MELEMENT        struct Melement
+ MELEMENT {
+diff --git a/raster3d/r3.showdspf/Ball.c b/raster3d/r3.showdspf/Ball.c
+index fb3b79882f..ae3ec3933a 100644
+--- a/raster3d/r3.showdspf/Ball.c
++++ b/raster3d/r3.showdspf/Ball.c
+@@ -7,8 +7,6 @@
+ #include "Ball.h"
+ #include "BallMath.h"
+ #include <stdio.h>
+-#define TRUE 1
+-#define FALSE 0
+ 
+ HMatrix mId = { {1, 0, 0, 0}
+ , {0, 1, 0, 0}
+diff --git a/raster3d/r3.showdspf/togif.c b/raster3d/r3.showdspf/togif.c
+index 589f243a8d..b48236b33f 100644
+--- a/raster3d/r3.showdspf/togif.c
++++ b/raster3d/r3.showdspf/togif.c
+@@ -324,9 +324,6 @@ static int ditherrow(unsigned short *r, unsigned short *g, unsigned short *b,
+  *
+  *****************************************************************************/
+ 
+-#define TRUE 1
+-#define FALSE 0
+-
+ 
+ /************************** BumpPixel() ********************************/
+ /*
+diff --git a/vector/v.hull/chull.c b/vector/v.hull/chull.c
+index 1ad97396fa..584af5d8ba 100644
+--- a/vector/v.hull/chull.c
++++ b/vector/v.hull/chull.c
+@@ -29,10 +29,6 @@
+ 
+ #include "globals.h"
+ 
+-/*Define Boolean type */
+-typedef enum
+-{ BFALSE, BTRUE } bool;
+-
+ /* Define vertex indices. */
+ #define X   0
+ #define Y   1
+@@ -76,10 +72,10 @@ struct tFaceStructure
+ };
+ 
+ /* Define flags */
+-#define ONHULL   	BTRUE
+-#define REMOVED  	BTRUE
+-#define VISIBLE  	BTRUE
+-#define PROCESSED	BTRUE
++#define ONHULL   	true
++#define REMOVED  	true
++#define VISIBLE  	true
++#define PROCESSED	true
+ 
+ /* Global variable definitions */
+ tVertex vertices = NULL;
+@@ -436,7 +432,7 @@ bool AddOne(tVertex p)
+     tFace f;
+     tEdge e, temp;
+     long int vol;
+-    bool vis = BFALSE;
++    bool vis = false;
+ 
+ 
+     /* Mark faces visible from p. */
+@@ -446,7 +442,7 @@ bool AddOne(tVertex p)
+ 
+ 	if (vol < 0) {
+ 	    f->visible = VISIBLE;
+-	    vis = BTRUE;
++	    vis = true;
+ 	}
+ 	f = f->next;
+     } while (f != faces);
+@@ -454,7 +450,7 @@ bool AddOne(tVertex p)
+     /* If no faces are visible from p, then p is inside the hull. */
+     if (!vis) {
+ 	p->onhull = !ONHULL;
+-	return BFALSE;
++	return false;
+     }
+ 
+     /* Mark edges in interior of visible region for deletion.
+@@ -470,7 +466,7 @@ bool AddOne(tVertex p)
+ 	    e->newface = MakeConeFace(e, p);
+ 	e = temp;
+     } while (e != edges);
+-    return BTRUE;
++    return true;
+ }
+ 
+ /*---------------------------------------------------------------------
diff --git a/srcpkgs/grass/template b/srcpkgs/grass/template
index bccd691a811..5863a61a300 100644
--- a/srcpkgs/grass/template
+++ b/srcpkgs/grass/template
@@ -1,7 +1,7 @@
 # Template file for 'grass'
 pkgname=grass
 version=7.6.1
-revision=4
+revision=5
 _binver=${version//./}
 _binver=${_binver:0:2}
 build_style=gnu-configure
@@ -17,7 +17,7 @@ license="GPL-2.0-or-later"
 homepage="https://grass.osgeo.org/"
 distfiles="https://grass.osgeo.org/grass${_binver}/source/${pkgname}-${version}.tar.gz"
 checksum=9e25c99cafd16ed8f5e2dca75b5a10dc2af0568dbedf3fc39f1c5a0a9c840b0b
-python_version=2 #unverified
+python_version=2
 nocross="tries to execute target binaries"
 
 post_install() {